Blizzard is for when they are landing or offstage. You don't do it in the neutral position. In the neutral position you sit back and approach (or not) with iceblocks. YOU HAVE A PROJECTILE, MARTH DOESN'T, HE IS FORCED TO APPROACH!!!!!! After he jumps because of ice blocks (because he eventually will...it might take 20-75 seconds) you can start getting under him and uair him. No, fair doesn't beat that option because fair isn't instantly below him, it takes ~10 frames to hit below him, and that hitbox is realy underwhelming. Counter will whiff if you space it right because uair is super disjointed. So yeah, blizzard when he is on the ledge and when you are above him landing since he can't do anythign about it assuming he is IN BLIZZARD RANGE WHEN YOU USE IT!!
Grab: Really bro? It isn't like Marth is punishable in like...every move he has when he is landing. It isn't like Marth's ground options are kinda terrible. It isn't like that if he keeps jumping we won't even HAVE to grab because of the damage we rack up from blizzards/ice blocks/uairs/nairs/bairs. Stop thinking with such a linear mindset. He has options for what you can do, but you have other options as well. Welcome to even or +1 MUs, you have answers for what each other can do, but since we are in the +1 position (IMO) we have more options than he does that cover more things.
Nair: You aren't using nair when he is in the ground. If he is on the ground, you ice block him into the air. Assuming the Marth isn't ********, he will jump to platforms to try and hit you for overcommitting. That's the wonderful thing about nair...you can't hit ICs out of them since the hitbox comes out so late and has relatively low cooldown compared to when the attack hits. You also aren't stretching out your arm, extending your vertical hurtbox. Nair on platforms is AMAZING, and it can lead to a grab (*Gasp* grab set-ups? You mean I can't just be ******** and blatantly throw them out all the time?)
So yeah, 4th reason is Ice Blocks. As I said before, Marth is always forced to approach when you are tied since we have projectiles and he doesn't.
Sorry if I sound a little angry, I've lectured about the Pikachu/Marth MU a lot to somebody that plays Leon and I get mad when people just say stupid **** about it when they don't actually comprehend the MU or when you actually have to think in a MU.