Ground control for Marth means limiting the opponents options on the ground from the neutral game, which goes back to your point about Marth's unique version of pressure. Marth's ability to cover options defensively is the focal aspect of what Taj refers to as "React and Deny" and it's probably the best simplistic approach to the character because you do get to cover many diverse options from the opponent. But Marth isn't like other top characters where you have a mix of aerial and grounded options because Marth rarely wants to attack a grounded opponent from the air outside of maybe sh fair walling and the opponent obviously has no business being in the air against Marth from a neutral standpoint. Since you're unlikely to engage a good opponent air to air, ground to air, or air to ground, the focal interaction from neutral is usually ground to ground. This is why marth's dashdance is so stupid, why dtilt is an amazing ground move, why FD get banned against the character a lot, etc.
This also means that a lot of the time, Marth has to dedicate a large amount of focus to circumventing crouching, even at mid %. Without controlling the opponents options on the ground, and without respecting crouching, Marth is not a good character. With them he's ridiculous. At your caliber of play, you do these both automatically. To some degree though, Marth can't always play around both the crouch game and the opponent's ground options because we're humans and we mess up and Marth struggles to reset to neutral if he's put into a disadvantageous position. That's fine, it happens. But it also means that your best shot of playing around them is to reset to neutral and then find a cheap way to circumvent crouching. To me, grab is the obvious way to do this. Marth doesn't have stand up > shine to reverse a bad position, and he's not really fast enough to weasel his way into something good all the time. Grabbing is your best bet to avoid crouching in the event that you "get them" and you want to be sure it doesn't bite you in the *** for a hit-confirm, where you can't always be so sure with the sword at low or mid %. It accounts for human error, which smash has plenty of at every level of play.
Hard to decide where to start here.....
Your first paragraph, my big issue with your argument comes in with you saying Marth won't aerial when the opponent is on the ground except for some SH Fair walling. You don't really go on to say why except in the second paragraph, so that's kinda confusing. I actually like rising Fair/Bair and spaced Bair(possibly)/Nair as they are all easy to maneuver with(due to aerial mobility+ground mobility giving great setups) and all tend to circumvent crouching well(at least on tipper). They can ALSO combo well, set up for grabs well(due to using not-grab moves that condition the opponent to look for grabs less), and in the case of Bair/Nair particularly, they beat out a lot of moves that grab might not have beaten in a similar situation.
Example: You have less stage control than the opposing Fox. Pivot grabbing is only going to be a one-shot deal, and any dashes backward that don't successfully dodge an attack leave Marth open to getting pressured/thrown offstage(not to the edge but if he gets lucky that happens sometimes). By using an attack in place/even retreating or sometimes approaching slightly, Marth secures a hitbox between him and his opponent. Marth takes control(if only some) of the situation. By only reacting in a situation that is inherently disadvantageous and not creating an opportunity, Marth is making his bad situation worse. He HAS to "guess" some, and grabbing in place/pivot grabbing/grabbing towards the opponent will generally only lead to a loss of stage at best, stock loss+momentum loss at worst.
Then this same example could be run when Marth has advantage, but I really want to get to your other point/make sure I have time to cover it. I'll suffice to say that Fair loses some of its disadvantages when the opponent is cornered.
No character can perfectly play around CC'ing, it is a ridiculous game mechanic. Even Peach can get wrecked off of it(from her double jab/some parts of her Nair/Bair and her dash attack). Even Falco can(his Nair/Jab/Ftilt/Uptilt kinda all suffer from CC'ing). I don't think characters change up their game drastically because of CC'ing, they just learn the CC'ing percents and then play to those percents/conditioning of their opponent so they won't CC as much/at all at those percents.
Marth can control ground options from the air. Nair in place tends to cover most lower angles, and some creative Fair'ing(particularly involving rising ones) tends to smack a lot of ground moves. If Marth jumps, he is CONTROLLING the opponent's movements right away. I personally want to learn different AERIAL zoning techniques for the sole purpose of controlling the opponent and tricking them into thinking I am about to commit to a laggy aerial instantly when I jump or when I land. That kind of power, combined with options such as FF'ing/no FF or WL'ing, can create a large, dynamic area that Marth can affect when he jumps. Have you ever seen Mango hit good players with SH drift towards them no FF Fair? It is because he conditions them to run away when he DASHES towards them, then they can't react to the jump in time to punish, so he gets to overtake them and then control/hit them with Fair. That is one way to use what I am talking about.
Marth may not have stellar wake up options, but that's okay. Every character has flaws. Not sure where grab comes into play to help that out though. It's pretty slow for the size of it's hitbox/whatever you call it and won't beat out the scary moves Marth has to deal with usually(except Fox Nair sometimes LMAO that's always fun).
I could have wrapped this up better but I g2g eat. Respond to this and we will work from there. =)
