Do Guard Bow, Distant Defense (A), and Distant Defense (seal) stack?

Yes all of these do stack. Any spur or in combat increase stack. But not hone or fortify.

They all stack. Buffs that you don't see on a unit are called in-combat buffs and stack with each other and with visible buffs that are colored blue on your unit if the unit revieves them. Those visible buffs can't be stacked with each other tough

Yes! My Faye (-DEF +RES) run this set and is fantastic against magic. VS physical distant damage is good too, but without a -def ban, she would run better