Well, what I found works is DI-ing sorta...down+left/right. This sends you low down to tech, or if you're at the ledge, prevents the typical knee follow-up unless he wants to risk a SD, and allows you to try to recover low. He's at positional advantage, but it eliminates his best kill option.
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From what Ive played in Marth v Falcon matches, youre right about DIing down + away on his throws onstage. The only problem with this is that his uthrow is good to combo with at low percents for fast fallers, even though Marth doesn't belong in this category he still becomes as easy to combo as they are if you DI down + away on a uthrow. Basically my point is to not get too comfortable DI -ing down + away every time you get grabbed.
At high percentages I think the knee is guaranteed out of a dthrow, Im pretty sure you wont have enough time to tech the stage before he reaches you. In this situation I think its best to DI up + towards falcon, even though the knee is still guaranteed, Falcon will have a very awkward time landing it, especially if your opponent doesn't expect it.
As for all his other moves, I would say to DI - Up + towards Falcon. Basically get above his head. Even though Marth doesn't like being above somebody, and Falcon isn't really disadvantaged here (he can still uair you) it is much better than being in front of the knee. Being above falcon uses his momentum against him as he has to continue stopping his natural flow, making things awkward.
So I would say anytime you see a dash attack, uair, sideB, or bair DI Up + towards. Never DI down or away on any of these moves (unless you are extremely close to the stage so you can tech) as it will put you in perfect position for more comboing. Think about it this way, Falcon has incredible horizontal speed; he will always catch you if you go left or right of him, but vertically he gets no special treatment.
If he hits you perfectly with a nair there's not much you can do, just buckle up and welcome the knee.
That's all I have to say about DI in this match up. For everything else regarding this fight, just keep in mind you have a sword and hes got arms and legs.