I'll try to answer your questions.
For the untrained or lead-footed driver, it's easy to drain the battery on manual transmission Civic hybrids and the Insight. I also think it's easy to take care of the hybrid battery pack to give them a good long life - just need to know what to do.
The software for the early Hondas did allowed the battery pack to go as low as 15% - too low. It was changed on later models and recalls to 40%. The key to long battery pack is avoiding deep discharges or overcharging. The 2000 Insight had the most problems as it was first. BTW, my battery pack was replaced at 94,000 miles. The new pack/software seldom does the dreaded recal - I almost have to intentionally do it.
Lithium batteries - that would be something for Mike Dabrowski (his site is at
www.99mpg.com) to attempt. I fully expect people like him to try lithium batteries and supercapacitors on the Insight in the coming years. He has improvised a 5th wheel for a limited EV mode, but added some conventional lead-acid batteries.
The Insight's 1st-generation IMA technology will not allow a true plugin conversion because it can't go in an EV mode. Still, anyone with the knowhow to beef up the limited hybrid battery pack would definitely improve the fuel economy and improve the emissions by reducing the load on the gas engine as it accelerates to highway speed.
I don't know of anyone yet that has put in lithium batteries as a kit, but it would definitely improve the fuel economy of an Insight, but not quite match a Prius with a plug-in kit.