Yep, both on hardtails. We have seat packs, bar bag for me and maybe a frame bag. Considering a rear rack, not for panniers but to strap compact sleeping bags/mats (i.e. little weight but some bulk) to the top platform.
Thermals and hat for sleeping!
food - bring a stove, pot, and whatever food you like to eat
bothy is next to a good water source, yes.
please pack as light as possible. you will be on MTBs? the route will be rather challenging if not