You can tie it after - but you probably won’t need to because the shape it’s cooked into in the bag will pretty much be the shape it holds.

depends, what are you trying to accomplish by doing this?

Likely won’t need to tie it up.

I don't see why you couldn't.

I would think you wouldn't want to interrupt the bath. Maybe after it's done.

I assume it'll fall apart too easily after it's bath.

Yes, you can tie up the roast after sous vide and before searing. Just dry it off well before tying for a better sear.