Minimum usage recommendations? (Prius semi-retirement)

12v battery on my 2007 Prius (w/ about 115k miles) died after a long vacation + an additional period of no use (so depleted it wouldn't keep a charge, even after a few "repair" cycles on a smart charger. Now that we have a solar photovoltaic system on the house, even if the Prius still uses little fuel, using a BEV makes more economic sense in terms of mp$. We were considering selling it off, but with the PG&E public safety power shutoffs in late summer/fall however, looks like we should continue to hold onto the Prius while it still works, as even if we had a home battery backup system, the PV system output (10-20kWh/day, depending on season) isn't nearly enough to get around much in a BEV on top of in house usage.

Looking for recommendations for a life support/semi-retirement level of usage/maintenance on a Prius that will keep it reasonably operational.
1. 12v battery, figure I need to put on a battery tender or run for an hour at least 1x/month if not more (or could I put it on one of those small battery tender solar panels. i know those cheapie panels don't have well controlled voltage, but could something like a buck transformer keep charging at a reasonable float level (13.3v?) that doesn't fry the battery even faster than not charging. Or do I need to shell out at least $10-20 for a proper solar/SLA controller that knows how to stop topping off?
2. traction battery was into the one purple bar zone when I got the new 12v battery a couple weeks later, and finally started it up again (probably 7-8 weeks non-operational). In some drivearound it did get back up to full green bars, but does letting the charge run down that much, repeatedly, damage hybrid battery long term?
3. the standard oil service intervals suggested of 6 mo/5k miles seems over conservative, even running it as my primary daily car I wasn't putting much more than 10k/yr on it. so if I rarely run it, do I still need to change oil just about as often as when I was running it daily??
4. the same type of question for the other fluid etc change intervals, the other time based intervals are virtually identical to what it would be with 10k miles/yr usage, so is there really that much degradation in the low usage state that it effectively makes no difference?
5. gleaning what I could from priuschat and other websites, looks like I don't want to keep around my fuel too long, so would it make sense to only fill up about a quarter tank at a time (~100ish miles each fill). Should I be keeping my gas tank between near empty and 1/4, 1/4 to half, half to 3/4, or 3/4 to full? More full seems like I'll always have a larger proportion of old fuel at any given point.

I'm guessing you wouldn't be able to stretch things out strictly proportional to mileage, especially at the extreme edges (let's say driving 40 miles/month ~500 miles/year your 5k intervals would be 10 years apart).. But what would be a reasonable degree to which the intervals could be stretched? Lets say my real annual mileage on it will be in the 1-2k miles range. (100 miles/month). 1/2 the frequency? (considering 1 year to be about 5k?) 1/4 the frequency (2 years = 5kmiles?)

Then am I best spreading the operational times out to a tiny bit at a time (25 mile per week?) or doing bigger drives less often (100 miles, once per month)? Or split the difference, 50 miles, every 2 weeks?

 

right that's why sorta of seeking what the life support level of operation would be. i've seen some ads for something like a hybrid battery tender here, but if a 20 mi/week or every other level of usage is enough to keep it in stasis, less that $400? outlay for the hybrid battery tender, plus some level of operation for the systems other than hybrid battery probably not a bad thing?

is the important thing to get it up to full charge periodically, or just minimize how much/long it gets into purple zone? i know li-ion actually doesn't like full full charge, ni-cd has the memory effect, but ni-mh, do you, or how often do you need full full charge?

 

that's what i saw, kinda pricey $480 right? cheaper than a new hybrid battery of course