2001 P3191 Troubles, 12V Battery related?

OK, I've been reading other posts for a while about P3191 and have tried quite a few things, but no posts that I have read quite match my situation, so here it goes:

I bought this 4-door sedan 2001 Prius with about 119K miles, that had a blown head gasket. Had no belt at the time, and was a steal for $1200 so I decided it would be fun to fix on. Almost flawless interior and exterior so, why not? Put the belt on right away. She's running great. I use some head gasket sealant with some coolant (which was pretty dry, hence the overheating), and this worked like a charm - tailpipe smoke cleared right up. She is still up and running fine, with no problems, better than ever. I didn't exactly bleed the coolant system, but I figured no need to yet. It is definitely on my list to-do.

Next thing to fix was the MFD, which was just yellow and showing nothing, although it would light up... so I bought one on the internet and swapped it out (I didn't program anything, as I don't think I need to, as I saw in a video this guy did not). The plastic that held it in place, which was the only interior damage and had a crack, totally shattered when I went to take it out. So I had to wait a few more weeks on one of those to come in the mail. During this time, she was not running, just because the dash was all opened up and the MFD was waiting for the new dash part to be installed. I decided to change the oil during this period of time (to get rid of that yucky brown chocolate milk coolant/oil). I swap the oil filter out while I'm at it. Which was the most stuck filter I had ever seen, man! But I managed to get it out.

Now the plot thickens. I finally try to fire her up after the MFD swap/oil change. But the battery had died. Uh oh. I think the owner told me the battery was on it's way out, that she had to jump start it or something (she was not a mechanical expert)... but she did contact the person she bought it from, and they said that they had replaced the hybrid battery pack already. Anyways, so I'm now able to start the car when I jump it, but not otherwise. The battery is not holding a charge. Every time I want it to start, I have to jump it again (I start using the emergency jump start battery thing). So there are two problems here 1) the 12V battery not working, and 2) the P3191 code.

I will go step-by-step through my interventions:

A) When it is fired up, it does the triangle of death thing and gives me a P3191 code. Engine sounds fine, in regards to the idle is not rough or anything, but then does the automatic shut off thing after about 8 seconds or so, as many people report. At first, I think maybe it's out of fuel. It was low. So I fill it with 5 gallons or so using a gas tank. But the new gasoline does not register. No changes in behavior.

B) I had noticed earlier that it took less oil then it said it would online. It says only almost 4 quarts, but I had just filled it until the dipstick read good. I thought... maybe it is not registering enough oil for some reason, as I read online that there's an oil weight sensor or something that can throw the P3191 code. So I fill it to the measurements I am reading online and it is over the dipstick amount. Probably not a good idea, as I think I might have caused a new problem. No changes. So I drain some of the oil out, and now it's right at the top bead on the dipstick.

C) Next, I clean the throttle body out, which had some build up but wasn't terribly bad. That managed to make the gasoline register! Which I think is significant, but it still won't stay on past 8 seconds, still gives me the triangle of death. It blinks as if there's nothing for 3-5 seconds or so, but then comes in with a full tank, before it shuts down the engine again. So that tells me that when the battery died, it reset the throttle calculation, and that was definitely halting the engine from starting. Oh, and also, when I let it stay on as long as possible, trying to pull the codes before it dies from no 12V battery functioning, the PS light goes on briefly near the end, right before it dies. Maybe that's related? Or something else? Still not letting the gas engine start.

But I should mention that in the process of taking off the intake, I notice above where the belt had been ripped off, it appeared to have melted/ripped through some of the flanges on this tube. Which was a pain in the butt because I had to get the headlamp loose and even the front passenger fender, but I bought a new one and put it in. I think this could be related to how it overheated? I don't know, but it was broke, so I replaced it. Here is the name of the kind of part I replaced: Toyota Prius W2 1.5 H 2007 RHD Air Intake Tube Pipe GF491-02670 15489782 (looks exactly the same as the Gen 1 part)

But basically, once I put this part in, and the throttle body is cleaned, I see a little progress in that the gas is now registering... but still getting my P3191 code, still getting triangle of death and stopping after 8 seconds. And yeah, that PS light comes on for a second or so right before it dies... not every time, but I've seen it twice.

D) I clean out the MAF sensor with some cleaner, as I figured maybe in the process of removing the air filter to clean the throttle body, that I dirtied something up. The filter looks pretty clean by the way, but it does have some slight tears on some of the flanges. I just left it in there, since that's how it was when it was starting, I figured that can't be the problem. MAF sensor is clean, still nothing changes. This whole time I am having to use my emergency starter battery pack to get the car started again, which will work for a few tries before it dies again and I have to re-connect it.

E) I brought the battery to O'Reilly's a couple times during this process - they said it is fine and charged it up. It did seem to be holding a decent charge, in that the emergency battery jumper kit thing no longer goes down from 100% anymore... at first it would go down a few percents trying to charge the battery. But for some reason I still have to use this emergency battery jumper. If I just connect the battery and try to use the key, there's no reaction. Just black all over the dash lights.

F) I'm in Hawaii, the Big Island, on the rainy side, and so corrosion is a definite issue. A friend of a friend is an ex-Toyota dealership person and they said they bet it's the hybrid battery, but won't really listen to all the details I want to give them. Then this guy at O'Reilly's the second time says every time he goes on vacation for a couple months he has to open up the hybrid battery and treat it for corrosion to get it started again, and that he has owned 5 Prius's (Prii?). So, I get it in my head this must be the problem and I open her up last Saturday, measure every module/cell (they are between 6.5 and 7 volts each), use the wire brush on all the terminals and bus linkages, along with electronics cleaner on them and the nuts, and then a light coating of dialectic grease on the buses, nuts, and the terminals. Frightened myself because then it threw a couple more codes, P3030 and P3077, when I tried to start it up. Apparently, I left the battery fan unplugged. Hopefully this will take care of both codes and I didn't mess anything up - I'll try to fire it up again tomorrow.

G) But today I was thinking, can it just be a bad 12V battery? This battery is not giving me a good reading when I use my voltmeter. It still won't do anything when it is connected, I'm only able to try and fire it when I am using the emergency jumper battery, connected to it via the red/black prongs. I took it to the Toyota dealership and they won't even test it when it isn't connected to my car (I don't trust the O'Reilly's, since they don't even sell these batteries). I could tow my car there and have them try it... or just shell out $400 for a new battery (they have to ship them to the island by boat apparently, everyone else is saying they are cheap).

My next steps/ideas:

a) try and clean out any excess oil that may have gone into the engine. I read somewhere that if it is overfilled, sometimes it needs to be cleaned out. I tried to understand where to access this but I can't really find any videos.

b) figure out if this battery is really dead, by checking for fuses or something? And if so, replace it. I'm thinking that if the emergency battery jumper works when connected to it, though, that possibly this means the battery system is functioning? Oh yeah, I did clean those battery terminals off, and the connections, with electronics cleaner.

c) clean that MAF sensor again. I watched a video and thought, maybe I could do a more thorough job. But when I did take it off, I didn't see any difference. Nothing looked dirty, nothing appeared any cleaner. But I figured I could spray that stuff way up in there again, just to be safe.

d) of course, I want to top off the coolant and bleed it properly. I think I can leave that sealant stuff in there... they say you can. I used only a quarter of the bottle, a very small amount. But I could change it out. Either way, I figured I would take it to the dealership for this, once I had it running.

Any ideas? PLEASE HELP! I've put so much energy into this baby already, I don't want to get burned out on it. I would really like to have it running soon, so I could either flip it and reap a little benefit for my troubles, or let my son drive it to school, who is getting his license very soon. But it definitely needs to be working reliably if I am to do that... Thank you in advance (or as they say here: Mahalo nui loa!)