Is it possible to edgecancel while facing towards the end of a platform?
Yes, you need to hold forward at the end to cancel the
Ottotto (ledgetumble) animation before it even starts.
So when edge guarding Sheik I noticed that when she up b's right where the edge is when you are holding it you are forced to roll dodge because of the flame hit box. I haven't tested it yet but would it be possible to just air dodge up with her then fast fall and grab the ledge? Or just jump a certain height from the ledge and then air dodge?
No, because as soon as you let go of the ledge, Sheik will grab it and punish you for wasting your second jump in that position.
Sheik can also grab the ledge while her flame hitbox is still out.
Ledge invincibility is enough to stay intangible until her flame hitbox is gone, but your ledgegrab timing needs to be close to perfect (not earlier than 7 frames before she could grab it from her ↑B’s first rising part).
Sheik can grab the ledge from her ↑B starting on
311 19. Thus, you need to grab the ledge within 18 frames after she starts her ↑B. Wavedashing while looking towards the stage takes at least 12 frames until you edgehog, so with only 6 frames remaining, we can count out edgehogging on reaction to her ↑B (6 frames reaction time is most likely impossible). You need to predict her ↑B timing in order to get the correct timing.
From a well-timed edgehog, you can cover all her options on reaction, I think. If she angles her explosion (I elaborated on that
here) towards the ledge or straight upwards, you roll one frame before your invincibility ends. If she angles it diagonally towards the stage, you ledgedash on reaction and punish her (fsmash comes out in time for reaction times below 17 frames).