READY flashes but gas gage is erroneously indicating EMPTY and ICE will not start nor switch to Drive/Reverse

Hello experts and the rest of us! I just figured out how to initiate a new post, so this one is mostly a duplicate to an earlier reply to someone else's posting that was similar in nature to my problem.
I actually have 3 Prius's (a 2002, a 2003, and a 2004, each currently inoperable for various reasons). I know my way around the battery system fairly well now, having rebuilt at least 4 of those battery packs as well as other times that I accessed the cells for troubleshooting.
My 2004 model has been the most reliable and we have registered it and keep in NY where we have a seasonal cabin. I have experienced numerous problems over the past 6-8 years due to the seasonal nature of its use and storage for months at a time, but I generally can get it back up and running each visit without too much trouble (it used to be the 12V battery was the main trouble due to something constantly discharging the battery very slowly, so now I disconnect the 12V battery when I am going to leave it for a month or more.) Having said that, I figured my 12V battery was the culprit again, having a 54% charge when I reconnected it this trip. So I charged it overnight and got it back up to 100% (per my rinky-dink battery charger). But the car still would not go into READY.
ButI figured the battery might still be the culprit after more troubleshooting in our remote dirt road location, and I got my brother-in-law to take me to Walmart for a replacement (just to mention, I gave up on the expensive pathetic Toyota mini-batteries years ago since they don't last long and are hard to access--I now have a standard car battery in the rear compartment that is easier to access when the battery is disconnected or dead--the rear hatch of course won't open without power to the latch lock! I did lose some trunk space, but it's well worth it with my multiple extended storage periods each year and has proven to be a very successful modification.) Anyways, the new battery did not fix the situation. And I even charged the new one overnight to get to 100% charge per my charger (the new battery was definitely not the freshest, being many months old per the sticker date).
Reading other posts on this site and others, I created a checklist of additional troubleshooting which I have now exhausted, short of going into the traction battery again. But I don't believe it is the traction battery this time--my energy gage is telling me it has 6 of 8 bars charge (about as high as it ever gets). I also am getting a clicking from the battery ECU when I hold the Start button down--coincidentally, the READY lite flashes when I hold the Start button down.
All fuses seem to be good--I haven't actually put a continuity tester on them, but they definitely aren't blown.
The oil level in the engine is good.
I've removed the negative battery cable for 10 minutes to up to 24 hours multiple times in the past week--that's supposed to reset the ECU, but not for me.
Short of the traction battery being a problem (unlikely in my opinion since that battery is only 2-3 years old now and it's registering good on the Energy panel), I think I may have a problem with the computer thinking the car is out of gas--that was the only other thing that some experts noted about the Prius. My gas gage IS INDEED blinking on the last one bar. I NEVER leave a vehicle in the winter tundra of NY with no gas in it, but it is possible that someone might have siphoned gas out in my absence. So I put the entire contents of a 5 gallon can of gas into the tank. STILL NO JOY, and the gas gage still blinks on the last bar. My experience is that the gas gage sometimes takes a little while to register a tank refill, especially if you don't put much in. So went and got another 5 gallons and this time the tank was overflowing with just the first gallon--so it is definitely full (perhaps too full, which might be another future problem based on some posts I've read here). My fuel gage is still blinking on the last single bar (not registering any additional fuel), so I'm thinking the stupid computer also thinks the car is empty and maybe that's what is causing the car not to go into READY??
I just learned that the fuel tank might have a bladder--what a dumb idea IMHO unless some designer thought the Prius might need to do barrel rolls or other inverted driving?!?!? So perhaps my bladder finally gave up due to the severe upstate NY winter weather? I hear that this winter wasn't that terrible since the Christmas Eve blizzard, which we were up here driving the Prius a few days later.
Is there some way to reset or replace the gas gage? That's all I can figure....