Can you tell me about getting in and out...
Can you tell me about getting in and out of the MontBell cover??Can I leave my sleeping bag inside the cover and roll it up to be put into a compression bag??
You certainly could do that, but I bet it'd be a big pain. (I've never tried it myself.) The cover is waterproof/breathable, so it's going to be difficult to get all of the air out when you're trying to stuff it.
BUT it's super easy to take your bag in and out of the cover, and the cover compresses by itself down quite small, so packing them separately shouldn't be a problem.
You could do that but I wouldn't recommend it. It is good to get your bag out of the bivy so that it can breathe and get any moisture and dampness out. Sometimes there can be a little bit of condensation on the inside of a bivy like happens on a tent fly.