Follow-up Q on Shield of Faith/Ricochet

By the same token, am I unable to play, say, Fleet-footed (even just for the own-dude-boost) if my opponent’s entire posse is protected by Onward Christian Soldiers?

Correct! This is because Fleet Footed is all one sentence and with the first sentence of any action being the requirement clause, Onward Christian Soldiers will prevent your opponent from playing Feet Footed