For DA, Falco can back up and laser if Marth looks like he wants to come in, or just back up and then aerial in if it seems like Marth commits in this way(run up PS or DA, etc). Also, while not terribly common, Falco could hold down vs DA and just not laser. In other words, the DA beating laser does make some assumptions about timing and how Falco sets up his lasers. That being said, in the current state of the meta, you're probably not going to have to worry about all of this terribly much and you should experiment with DA. This is one of the two matchups where I'd even kind of recommend the move at all and this is the best matchup for it I think. I'm not necessarily against it, but I think doing it a lot like M2K wants to do is a mistake.
For the shield, Falco cannot threaten anything OOS. Your Fair beats everything. He could FH, but if you're close you can just FH/DJ with him and catch him with Fair or fall and punish his landing if he pops DJ early etc. He could WD back which is a fine enough mixup, and if he mixes this and FH well he can sometimes get away with WD in or Dair OOS or roll either way. But really, the position is pretty terrible for him since he's slow and does not have big range. Let's also remember that shields aren't infinite and he only has so much time to act, even if you never attack his shield at all. You running in to grab means giving up your range and mobility advantage that I've just described. The reward can be worth it sometimes and I don't wish to say anything impractical, but tunneling on the grab is a massive error that ignores Marth's major strengths. BIG SWORD BIG FAST
I'd recommend instead that you occasionally poke if you want, but really begin learning how to beat/set up on his different OOS actions, how to condition with slight movement and spacing differences, and really to begin thinking overall of how to abuse Marth's superior power in the position. Practicing the position and really looking at the options will help a lot I think, as well as picking Falco and reversing it.
I rarely advocate for take laser side B. Or rather, I might dash/beginning of run side B between two lasers/during some other Falco adjustment period but not exactly am I interested in using it to counter anything or to "hope" it works. It's great at hitting high and so is great for when Falco jumps and especially when he lasers since it takes a while to come out. Oh let me explain it like this. First I think you should slow down Falco so he's not doing laser into aerial or laser dash back aerial so much. Take laser jab will beat this at an important spacing(s). It beats Dair and Nair when tippered but you have to do it right away(you can second jab to beat them waiting/dashing back then attacking more on reaction I find...but if they keep waiting without laser it gets weird lol). Then once you have him either switching his laser timing up OR doing 2+ lasers in a row before attacking, you can now take laser run side B on reaction because you can now be primed for his different reactions.
You might need to practice it, or you might need to be closer. Also opposite end of platform is pretty different on FoD as compared to DL lol. Staying just out of his Dair OOS range, so pretty close to him is what you want to do. He can't really do anything here as I outlined before. Maybe practice from this closer spacing and see if that helps and we can talk again if not.
Well it's kinda weird. See, Falco NEEDS to laser. He literally cannot function without it because they make up for his terrible speed and range. But, it is a big commitment on the front end to get it to come out. Because of this and the fact that he jumps when lasering, he gives up control and safety in order to get safety. So he moves the safety somewhere else basically. He could move it backward like with a reverse laser, or he could fall from a platform to mitigate laser jumping startup, and/or he can change timing/other spacings on his laser(such as different drift or waiting for new opponent position) to change his laser. This all sounds pretty nifty, but in practice Falcos aren't so complicated unfortunately. I find that when I stop their initial laser plan, they often will try to yolo a little more under pressure. Then switch to something odd where it seems like more lasering and erratically moving back and forth between yolo and more lasering. I don't have the full mechanisms of control nailed down on this yet but it seems that's the overarching archetype Falcos use. That might be more useful than the earlier stuff I said anyway lol. Fkin Falco