This is going to be a looong thread, I think...
Unless you have strong reason to suspect the ethics of the dealership, the car should be good. The fact that it seems to have a new 12 volt battery is a good thing, not a bad one. Some cars have their capacity bars reset at auction when they have lost one or more, but a 2015 with 10,000 miles is not likely to be one of those. If you can find out, via Carfax, where the car originally resided, then we can tell you, going by the climate there, how likely the battery is to be close to losing that 12th capacity bar.
Make sure that the extended warranty covers the climate control system, or it isn't worth the money. It will NOT, even if a salesperson tells you otherwise, cover the battery capacity.
Getting a second key and having it programmed for the car will cost you about $300. If the warranty costs more than $1000, I suggest you bank that money instead and buy a second key.
Scarlet Ember 2018 Leaf SL W/ Pro Pilot
2009 Vectrix VX-1 W/18 Leaf modules, & 3 EZIP E-bicycles.
BAFX OBDII Dongle
PLEASE don't PM me with Leaf questions. Just post in the topic that seems most appropriate.