You are getting shield grabbed because you are approaching aggressively with nair and misspaced fairs. You seem to have a common marth habit which is to nair - DD - nair - etc. Nair in place is really good as you see based on your 2nd nair in part 1 vs falcon. Aggressive nair gets punished harshly. Clearly you are so much better than this guy that you can get away with just rushing in and making it work but your approaches are heavily punishable.
Work on edgeguarding. Don't jump out with fair unless they are completely out of options, and even then there are better options. Especially vs cfalcon, you need to be killing when you get them off stage a serious majority of the time. I'm not sure how you aren't connecting more fsmashes, or if he shamelessly recovers high, stick him for a ton of damage for it.
Meanwhile, you should pay alot of attention to your recovery. If you recover on stage floating down like you do at, say, 2:33 in part 2 or at 6:15 in grand finals you should be dead every time. You need to save the jump, don't instinctively double jump when you get hit, which you also do on stage a lot, and float down off stage watching the opponent carefully. If they try to come out at you you need to fair them away. WHENEVER POSSIBLE, recover by sweet spotting the ledge with your double jump from below.
Edit: Also ledge game:
When he is on the ledge - be more patient. Space for him coming up, and look for a GRAB. People coming up off the ledge are very easy to grab.
When you are on the ledge - no matter how long you stall, marth is too slow coming up that it is obvious when you are actually coming. If you see him spacing for you to waveland towards him or come up with an aerial, waveland in place right near the ledge or simply hit over and stand up. Another thing you can do is stall HIGH, as in unsafely accept that since he's spacing for you to come on stage at him, he can't reach you from there. Again, you have to survey the situation each time. But if he's spacing for you to aerial don't force it.
