If you clan mates send def to ur stronghold then indeed they will gain xp and pvp points too help ur team mates out more at defending your base you should be upgrading your base defences for them as for stashes no one can assist you on the stashes as only you can see and claim that stash so unfortunately they can't help you on the stashes but at your stronghold for sure.

If a clanmate were to send reinforcements to my stronghold or a stash that I hold, would he or she gain xp for any troops that he killed while protecting me?

If not, does anyone get the xp?

yus they would, but make sure it's with the event, no fun killing good troops with no event to give you extra reward