So, this is just going off of stuff that has been exhibited before and the little bit of work we've done in the Discord mixed in with more basic general theorycrafting, BUT:
If Marth makes the mistake of shielding when you laser in, then grab is free. A grab on a low % Marth means you can uthrow fair for like... I think 30-40%, which is pretty nice. Marth's "10 mile long sword" has one very simple weakness: no lasting hitboxes. If you can laser and then DD right in front of him (but out of fsmash/JC grab range) and force him to whiff something, then you should be able to then create an opening from it on reaction. Also, Marth's OoS options are pretty bad overall, so if he shields an approach, SHFFL nair shinegrab should be pretty easy money, and that goes back to the uthrow stuff I mentioned before.
(People can correct me here if I'm wrong,) but from the bit of testing I've done so far, Marth is possible but hard to pillar. Aerial -> WD out -> almost instant DJ -> FF through with the late hit of dair. AFAIK the first time or two you do that you can get a shine if you're really smooth, but utilt is way easier.
Recovering past Marth is literally the bane of low-level Falco's. Basically, you need to mix them up as hard as you can. DJ to a position where you'd normally sideB and then upB choosing a weird angle, get a good grasp of M2K angles (ride wall for sweetspot) and Mangle's (going high and FFing to edge around their hitbox), and even do some high shortened sideBs to mix them up. Figure out where/when you're able to sideB edgecancel as your recovery and mix that in to your play, too.
Once you've got the edge, it's still not easy. If they're not expecting it, <100% getup attack is a pretty effective tool, and DJ falling dair from ledge can work very rarely, but the main mixup you'll want to have control over is ledgedash shine/utilt vs double lasers from ledge. The basic idea here is that if they respect your ledgedash and give you that amount of stage back for free, then they're almost definitely gonna give you enough room to get the bit of damage put on by the lasers. As one last mixup, you can also add in ledgehop waveland on side platforms sometimes.
Random addition: once you're on ledge, mixing up how many times you refresh invincibility (if at all) will probably mess with a lot of Marth's. If you can firebird stall for like 30 seconds and lure them into a false sense of security, it may be easier to catch them off guard and make it into an opening.