If my house had 2 busted galvanized pipes in the attic due to a freeze, and I file a claim with my HO insurance....
is it "normal" for them to cover the cost of repiping (with PEX or other) the whole house, say 15 drops?
Or will they ONLY pay for the cost of fixing the 2 leaks?

My experience with insurance is that they will only pay for what is broken and in your case would not pay to replace pipe that didn't have any damage to it.

They will only pay to replace the entire item if it's something like metal siding, that doesn't exist anymore and can't be repaired. If it can be replaced or repaired they won't pay.

Exceptions are things like some things like aluminum wire and some dangerous electrical stuff, if it's found and a risk though I'm not totally sure what would trigger that as a claim exactly.