Update: Took it to Nikon Service Center.
Tech said main board needs to be replaced. - $380 job.
Not covered under warranty but "complementary" because I used a third party battery. While I am grateful it was not "impact damage" - I don't think it is the battery.
According to tech, battery might not provide enough power and damage main board. He specifically mentioned Live View drawing more power and being damaged because of it. Doesn't sound right to me but I am not going to argue while I'm there.
Maybe Nikon designed their cameras differently, but I haven't dealt with any electronics failing because of insufficient power (of course I could be wrong - I mean at least not because the battery "couldn't provide enough power").
I have gone through about 20 aftermarket batts and have had basically no problems with any of them (on Canon systems). Maybe 2 died early, but not sure if it was because of the freezing temps I used them in.
Never had one issue with the 10 Canon bodies I've bought and sold (T2i/T3i/60D/6D/5D3).
Owned Nikon for almost one month and been to the Service center 3 times now. Also got "blamed".
I will try to do a test on the battery and purchase an OEM battery and compare.
Admittedly not a battery expert, but assuming if I test the individual cells by recharging and discharging at 1C it should tell me something?
I have a turnigy 6s charger lying around. I think EN-EL15 is 2x 18500 cells?
Min discharge voltage should be around 2.75V right?