Answer to "Will 6 Plus battery work on 6s plus"I have done this before: installing 6 Plus’ battery in a 6S Plus. Battery life was slightly more but Apple Pay would not function properly (authenticated on the iPhone but unable to detect by the cashier’s terminal).
Answer to "NFC reader not working after replacing battery."I had replaced my iPhone 6S Plus battery (2750 mAH) with that of 6 Plus (2915 mAH) since both have the same connectors. Battery life did slightly increase but at the expense of the non-functioning NFC/Apple Pay. I think it might have to do with the overclocking of the NFC frequencies out of the acceptable spectrum . I thought the slightly longer battery life wasn’t worth the lack of NFC so I installed the correct one for iPhone 6S Plus battery (2750 mAH) and NFC/Apple Pay is working well afterwards.