So yeah this one is gonna be long. Theres just so much things you can do edgeguard, it's ridiculous. Here you go
Marth- Ledge hop back air is super good, especially if he doesn't have his jump. Dair trades with up b, so if you have a good read on when he up bs, you can shine turnaround downair. You can also stand on the stage, look for an opportunity to SH dair him, or runoff dair, trade with up. You can also full jump, then down with a dair to cover up b to ledge. Marth has like 30 frames of lag on his up b, so you should be able to react, fastfall, then dair. Standing on platform and coming down with a dair is super good too, and is way better, as long as you're not on FD or stadium. You can also standback a bit to avoid up b hitbox and then wavedash in dsmash, which, as long as you react quickly, will cover both options. I haven't tested that out yet, as I thought of it today, so
I'm not sure. An alternative that I know works, and works really freaking well, is to stand back a bit, then dsmash/fsmash if he comes on stage, and run foward SH dair if he goes to ledge. Always mix in run off SH dairs and on stage SH dairs if you feel like you have a good read on his recovery. As you can tell from this guide, edgeguard marth from on-stage is a lot more complicated than edge guard from the ledge, but Imo it is better if you do it correctly. I really enjoy edgeguarding marth with falco, theres so much more to it than other characters vs marth. They're all just the same, basic, boring, grab ledge, force on stage, punish flow charty edgeguards. Also, you can invincible ledge dash downsmash, which is super good. Find what works best for you, you just need moderately quick reactions and a good read on his recovery. I personally prefer standing above on a platform and then coming down with a down air, but it all depends on the person. Also, suicide down air kills 100% of the time if you do it right, theres nothing he can do to stop it, even with a jump. Just remember that.