I think it's even- or if it's Marth's advantage not enough to warrant a +1.
I agree w/ everything you said thus far. However, even though Marth's juggling ability is better than Falco's, Marth is going to be forced to take a position where he runs the risk of getting juggled more often than Falco. Marth is going to be jumping in at you and the slightest mistake for landing fair in spacing gets you a grab (or jab nonsense on PS which leads to grabs). From there Marth is the the air getting juggled. If you shield a rising fair, the option to usmash is there if the spacing is right. Granted, strictly theorycraft, you can say Marth won't space fair badly, but realistically it does happen.
In neutral, like you said, Falco controls space a LOT better than Marth due to lasers meaning that you get to control the pace of the match. At low %s, that's a huge problem because a grab means we're offstage (a grab starting at a wide range of %s unlike our grab which is 0-4) and you get to rack up so much damage from there since Marth is so bad on the ledge- especially w/ RCO.
Falco also seems to be better at killing to counteract our advantage in gimping. You force Marth to play your game with lasers/sideB and we need to act around them. As a result the openings for you to kill us are going to happen more often than the other way around- especially with options as quick as your jab to cover mistakes that we can only mimic w/ upB which has the obvious downside on whiff.
Honestly one of the biggest reasons I think it's even instead of Marth's advantage is because of how good Falco is from the ledge. Marth gets so much damage vs every character (especially top tiers like Olimar and Snake) in his ledge traps that having a character that can circumvent them with such a number of strong options (nair->stuff on shield is difficult to punish outside of upB, and I know mine has been PS'd before, phantasm is annoying, your ledge jump is so high that we can't cover it w/o abandoning other option coverage, dair the back of our shield is essentially safe, again sans upB) is really difficult to deal with.
And while this part is fairly minor in my opinion overall, I also feel like Falco is such a solid character with a few traits that are absurd that his ability for a comeback when down vs Marth is more consistent than vice-versa as a comeback from Marth would mean needing to play your game the entire time and hinge largely on the death grab. Conversely, if Falco is down, he can still play his game to rack up damage slowly while Marth has to struggle to avoid it all- this results in him still probably approaching into any of your strong defensive options.
I can certainly understand the opinion of it being Marth's advantage and I'm not inherently against the idea. I just know most of the Marth panel (maybe all? idr) wants to bring Falco up for discussion so I'm just seeing how people feel about it who's opinions I actually care about =)