Engine ticking - low MPG - HV Bat not getting a full charge

I picked up a cheap Gen 2 as a backup. When the Amazon driver folded my two left doors forward on my main car, I pressed this one into service early.
I've replaced the rear struts, put my good tires on it and had to replace the windshield. It works, but there is a ticking sound. Sounds like valve lash ticking, but it comes and goes. Mostly comes, but sometimes it stops while the engine is running. Is there anything NOT valve related that would make that sound? The fact that it disappears at times raises that question.
Second issue is that the engine likes to run most of the time. I'm used to "feathering" the throttle to get it to run off the HV Battery, but this one doesn't like that mode much and mostly just runs. Mostly... Oh, and it acts like the "the transmission is slipping". If I accelerate strongly, when I hit a certain speed, the engine goes into high RPM and I let off the gas to reduce the RPM. Then it will still accelerate up to 70 without overrevving the engine. But it takes a bit of finesse.
Third, the HV battery doesn't get a full charge most of the time. I've switched HV batteries with the same outcome. Yes, they're both old, but I've reconditioned many of them and have had good results always...except this time. I don't think it's the battery(s). I have had the battery take the almost full charge during a mostly downhill time which tells me that the battery will take the charge if given the opportunity. Something up with some computer related something, I'm guessing.
And finally the only codes I'm getting are:
P0137 Oxygen Sensor Circuit Low Voltage (Bank 1 Sensor 2)
P0420 Catalyst System Efficiency Below Threshold (Bank 1)

 

She's got 282205 on her.
Code reader is a Foxwell NT716 Four System Scanner
And I'm just saying that it doesn't look like it is usually charging up over half. But it CAN given the right (downhill) stretch.

 

And, one of those batteries just came out of my other car that was working right, /so I feel confident it is NOT the battery..

 

I figured I'd ask here first. Those don't look to me like they could cause those issues, but hey, I'm no expert. That's why I come here with my problems.

 

I apologize for missing this comment earlier. Very interesting idea. And yes, the ticking changes with engine speed.
But here's an update:

The ticking got louder and louder. Then on Dec 1, while delivering packages, the ticking got even louder. Then the engine started to misfire. I pulled into a driveway to deliver a package and about halfway up their gravel driveway, it just quit. Done.

Then a little smoke came out of under the hood by the windshield that I recently replaced. Opening the hood, I discovered some small flames down behind the engine like under the driver's feet. Throwing snow back there didn't help. Long story short, the fire department arrived 30 minutes later after the whole car had burned up. I think the rear tires might still be good, but that's about it. I gave it to my friend that deals in Priuses for towing it away.

 

Guilty of not being clear as charged. Under the driver's feet was correct though. Which means way down below the inverter or next to the transmission. It was definitely near the ground. As for gas leaking, while I did not smell that, I was only getting 20 to 25 MPG. But I chalked that up to what was acting like a slipping transmission and its refusal to charge the HV battery and run on it during low energy needed moments.

As I think about it, when it started misfiring, did the exhaust get louder? Or was it just that I started paying closer attention to the sounds? Not sure, but exhaust leak and gas leak combination? On the left side? I don't think so. Improper locations, right? Fire definitely under driver's feet.

Bottom line is that this car had serious problems that I had hoped to solve by replacing the engine/trans as a unit, but it didn't make it that far. However, it was cheap, and I have already been reimbursed in full by the Post Office with the mileage allowance I get. And I made it all the way to 81 before ever burning up a car.

It would be nice to know what happened though.