Plumbing – Can you use PEX for venting

Yes you can do that (it's the standard way of converting to/from copper), and it shouldn't cause any troubles (read Why would you use copper over CPVC or PEX? and Does Pex tubing have any advantages over copper? for some comparisons).

Of note: you can get the male adapters that fit on the outside of copper pipe, and also that fit on the inside of copper fittings (like a street elbow). Can be handy sometimes if you're working in a tight space, or were going to immediately make a turn anyways (since you're soldering the fitting anyways, it's barely any extra work to do an elbow+adapter, and saves a PEX elbow and couple of crimp rings).

Yes. In fact, I had to do this exact same thing in a house in MN that had baseboard copper freeze and burst in an attic. I ended up replacing the cooper between the boiler and the baseboards with PEX.

I'd suggest using SharkBite connectors to transition between the copper and PEX. They're designed for that and it's super-easy to install.

As Ecnerwal points out, be sure to use PEX with an Oxygen Barrier.

While doing this, be sure to fully insulate the PEX. PEX can withstand freezing, but it's best to avoid freezing in the first place. If you can move it into conditioned space, that'd be best. If not, wrap it in quality pipe insulation (not the cheap stuff but the ones that self-seal).

The biggest challenge might be finding just the small amount of PEX needed as they are often sold in large rolls. You may want to check with a plumbing supply warehouse to see if they'll sell you a remnant piece.