It's the same failure strategy as always, is there some magical way to spend least amount possible and end up with an equally acceptable result?
No, and if that were the case, they'd degrade the lesser batteries to fit their price point which is what they did.
Your post about theory of what happened and what to do, is crazy and probably wrong and may burn your vehicle in a fire.
You need to get someone competent to look at it. If it's not worth more time or money, then it is time to get scrap value out of it.
Older vehicles incur losses, from wiring/connectors, starter, and ignition system. They need a higher CCA battery more than a newer one does, not your opposite strategy that his is a place to skimp. If you want to buy generic parts for repairs I can understand that (to a point until it's about safety) but battery is not an area where you can't fix the other electrical system issues to get equal life out of it, even in the best of environments.
If it is not worth the bother, you've already decided it is not worth the bother... do you get my drift?
At the same time it is a substantial % savings in cost. You're just trading doing more work to come back later and if you want to scrap it soon, not coming back later could make sense.