why did it take an ex Bacon Q400 crew to refuse?
Well, it didn't. This subject first came to light about five years ago when a flybe crew refused to set sail for Bergamo for the same reason. At the time, a procedure was worked out that satisfied the CAA and the crews (can't remember what it was, I was flying the aircraft with O2 in the cabin so didn't really care).
Of course I would have to agree that ordering the Q400 without cabin 02, steep approach mod etc was a bit short-sighted, but there you go...