The FAQ entry for Rexler Brath reads as:

...Any faceup damage cards that have been resolved and flipped facedown and any additional Damage cards dealt by the Minor Explosion damage card are also flipped faceup when Rexler Brath uses his ability...

So the question is this:

Say that Major Explosion (faceup) is one of the initial cards that is dealt, gets resolved, deals another crit (From rolling a Hit result, particularly one that is then flipped facedown), then both are flipped facedown... His FAQ entry specifically references additional damage cards dealt by the Minor Explosion damage card, but would it also work for additional damage card that would have been added by Major Explosion?

Yes.

Oh yes I've wrecked many a large ship like this.

Doesn't it flip the explosion faceup again so you roll again for the second?

Yes, all the card dealt will all be turned face up. FAQ is not always literally everything. Sometime this is an example list that you extrapolate.