Yep, you guys get the problems with ACaEBG pretty well.
In order to balance it, you either need to add some prerequisites to the power or provide some method of countering it.
I favour the latter method, partly because I think that a Changeling Knight Of The Cross is a good concept for a character and I'd rather it be possible.
Also because I think that gods and upper-level Outsiders and Angels ought to have something along the lines of ACaEBG.
Ideally, Mab and Titania would both have an ACaEBG-equivalent and a defence against the same. That way, their power can be modelled without making them unable to fight each other.
PS: Taking Sacred Guardian and Weapons stunts with a Sword Of The Cross doesn't make you a munchkin. That's exactly what you'd expect a Knight to have, narratively speaking. And all Knights have True Aim. Making that combo broken causes non-powergamers to make broken characters by accident.
PPS: I'd expect the problems with ACaEBG to set in around 13 Refresh, as a ballpark figure. Since Knights need to waste some Refresh to qualify for ACaEBG, the problems can be disguised for a while.