[Ecky]

That's odd with the battery, but it tells me that no, it's (probably) not a catastrophic failure after all. Mine never failed so I didn't get to see the full gamut of symptoms, but I understand that assist should go first, then auto-stop as it gets worse, followed by not being able to start with the IMA.

A very failing battery can cause a very small amount of constant background charging which can hurt mileage. Using a calpod (clutch) switch eliminates that, and can allow you to keep auto-stop and the ability to start with any but a completely hosed battery, as you won't be stressing it with assist and regen and the car won't figure out how weak it is. After a certain point 12v charging will shut off too, until you turn off the breaker in the back and unplug that computer. My guess is that a grid charging might bring it back for a few days or weeks, and you might get months or even a few years out of it so long as you prevent any assist/regen with a calpod switch. Up to you about what you do.

If you want to leave the option to go lithium on the table, you'll probably need to keep the pack, or at the very least the computers on it and maybe some of the sensors. I suppose you can probably get a cheap dead pack at a later date though should you choose to go down that path.

Charging at idle is affected without the IMA. I don't think the car will die if left idling in your driveway, but the DC-DC won't be able to get the battery up to 14v (fully charged) with the engine a 900rpm.

With the stock springs, probably between 1/4 and 1/2 of a coil to take it down an inch or inch and a half.. I'd measure ride height before taking anything out of the back, then cut a quarter of a coil, drive it around a bit then measure again and see how close you are. It's deceptively small, how little you need to remove to lower the car by an inch.