Can I disarm a unarmed zombie? You know.. disARM it?. Included in case I can.
I'm going to have to say no, certainly not with a rapier. Maybe if you used the scimitar I'd allow it for flavor but his attacks would be mechanically the same. Indeed if videogames have taught us anything it's that one armed zombies are just as deadly as the two armed variety.
Edit: Actually since you rolled minimum damage anyway, why don't we just say you used the scimitar for that attack and it can happen as you described?
We'll just go with the first roll. I don't think disadvantage is really necessary in this case. Milo's focus is on Chin and he's not really moving that fast so once Autumn actually gets to him grabbing him isn't that difficult.
Autumn is going to do 2 things. The first is using a spell slot for his ability, Primeval Awareness:
Beginning at 3rd level, you can use your action and expend one ranger spell slot to focus your awareness on the region around you. For 1 minute per level of the spell slot you expend, you can sense whether the following types of creatures are present within 1 mile of you (or within up to 6 miles if you are in your favored terrain) - Aberrations, Celestials, Dragons, Elementals, Fey, Fiends, and Undead. This feature doesn’t reveal the creatures’ location or number.
After checking for these sorts of creatures nearby, Autumn is going to try to use a Nature check to see if "fire-setting" could work to break down the tough rock: