Default priority for 2960S/X and 3850 to my knowledge is 1, not 15. So as long as your master has the priority as 15 you should be fine. What model switch are we talking about?
Does 8.2.121.0 also support the -B regulatory domain access points? I know there is a note specifically calling out support for the -B regulatory domain support in the 8.2.111.0 release notes, but did not see that note in the 8.2.121.0 release notes...
Is the switch you inserted the NM-4-10G into in 3850#2 a 48 port or 24 port switch? The 24 port 3850 does not support the NM-4-10G and will only support the NM-2-10G and NM-4-1G.
To manually upgrade via cli you either need to have the file loaded onto a USB key that the switch will recognize or on a tftp server on the network
!usb flash upgrade:
archive download-sw /overwrite /imageonly usbflash0:name_of_tar_image_file.tar
...
I believe in order to add a 2960X into a contiguous stack of 2960S switches you have to force the stack port speed on a 2960X to 10Gbps via the command switch stack port-speed 10 according to this guide you MUST set this manually, as the switch won't...