You can release the edge on frame 9 (8 frames lag) in PM iirc, and you are vulnerable on frame 38 (37 frames invinc total). So you get 29 frames of invinc to work with. At least one of those has to go to the double jump start-up. But when a character counts as above the stage during a plain double-jump forward from the edge (and thus when they can air dodge backwards at a downward angle to land and slide off in consecutive frames) is determined almost entirely by their ECB, Environment Collision Box, and how their double-jump changes their momentum (and thus position). I'm pretty sure Melee's vets in PM all have matching ECBs between games, assuming that's what PMBR means when they say landing detection is the same. But it's likely the case that the one-frame physics delay causes character's Hax Dashes to be delayed by one frame, as both the double jump and air dodge momentum will have be delayed by a frame despite their animations occurring at the right time. So any characters with a Hax Dash that's only invincible in Melee if you are frame perfect will likely not have that in PM, they would be vulnerable for a frame, if I am correct in my assumptions.
As for newcomers to brawl, I haven't a clue. They probably use Meleeified ECBs for their animations, but you'd need more details than that to know for sure, or some testing at the least.
