Swagic
A useful thing to know is that any time Marth forward smashes your shield--even if it's sweetspotted--you have a guaranteed followup! Most mains know about the followup when it isn't. When it isn't sweetspotted, WD OOS grab always works. I don't know how well-known the followup for sweetspotted fsmash is. When it is sweetspotted and you're knocked backward in shield, WD OOS ftilt always works! Tested it a few days ago at the max possible spacing. The caveat to all this is that your wavedash needs to be full length, something that comes with practice. But anyway, the takeaway is: any time you and Marth are on the same height and he fsmashes your shield, you get guaranteed free damage!
With regard to Ganon's upB: It actually comes out on frame 13, not frame 14. The massive grabbox is only out on 13 before it shrinks to a much smaller one on 14. (See the frame data thread for a gif.) And yes, it is a true teleport. If you have 20XX, you can see the magic yourself by advancing frame by frame. Ganon will be crouched on the ground for 12 frames, then suddenly teleport forward and hug the opponent if they were in-range. If they were at max range, Ganon will appear quite a distance from where he was crouching just a frame before.
I'm not sure what you mean about upB as a counter to Peach's dsmash, though, since for the first 12 frames of startup you're completely vulnerable and therefore would get scooped up in the dsmash if you tried upB OOS.