Manly Oval was actually meant to be our home ground, Unionites in Manly Council refused us permission to play there and Warringah Council offered us Brookvale Oval to use. A proper rectangular stadium in Manly would have been awesome. It'd turn the whole suburb into an unusable carpark on game day, but it'd actually be pretty easy for opposition fans to access via ferry.
Rat Park has the land to build a proper stadium, it's just another 20 minutes further up the Beaches and would make the travel times for opposition fans very unappealing. It otherwise could work well, you could build a huge carpark, proper facilities etc.
Chatswood Oval would be good if we ever get the Chatswood to Dee Why rail line. You could be in and out of the stadium really quickly. There isn't much room to build there though.
It's been done.
You're right, in a sense, that the merger of the territories shouldn't have been allowed to fall apart. Whoever came out of the other side of the Northern Eagles falling apart should have also assumed control of the development of the other's territory and junior systems.