I'm not an engineer, so I can't really have a discussion on your level.
I'm just an amateur who likes to play with electricity.
I think to even talk solar, one needs to figure out the drain of the accessories. Anyway, maybe one day it can be incorporated to help with the entire system....
I think battery prices for the car market may come down a little if Tesla's Giga-plant comes to fruition. I believe it's Panasonic that supplies the cells to Tesla, which means Panasonic is in it for the profits and even though the two companies probably have quite the deal, Panasonic still make out. If Tesla can make their own cells, then the profit margin will be slashed, probably bringing the cost of the car down.