I also think Heather's donning the costume was inevitable, and Byrne probably begged off the idea to avoid spoliers. Heather toook the leadership role, and was hurt in battle almost immediately with the Plodex. I started reading Alpha when a cane-reliant Heather was leading the team, and it was a very distinctive take on a super-team. Then in AF #24, Heather stood bored while the others fought the Great Beasts. I think even if JB had stayed on the book, we would have seen Heather in the suit. Byrne did design the sunglasses Heather wore that added so much to Heather's suit. I'm inclined to believe that Mantlo picked up directly on that thread from Byrne, with Jeffries and Box reconstructing the suit fron Courtney's remains.