I have been using a mummy style sleeping bag for a while now in my hammocks. Here's what I have found. They are a real pain to get into once in the hammock. This last trip I decided I would change up my routine, and I basically got into the bag standing up, and then got in the hammock. It made it much easier and less hassle. You need to put down a tarp or something to stand on so you don't get your bag wet. I also tossed my jacket over the hammock (no bug net) and then once I was in the hammock and comfy I would pull the jacket down to cover my head where the opening was in the bag. While I am not claustrophobic I did have a moment when I woke up with the urge to use the restroom, and couldn't see anything. Once I woke up and found the zipper I was fine, but being encased in a bag and having your head partially covered can cause anyone to panic.
I also used some of those hand warming packs they sell everywhere now for the first time. I found that if you placed one on your neck right it really helped warm me up it seemed. It could be just an illusion because of the amount of heat vs. what it really takes to heat your body, but I slept pretty good with it. I would wake up occasionally and switch it to the other side of my neck.
Back to mummy bags, I am actually going to try and find a rectangular bag because I think the mummy bag is just a little too restrictive for my movement. I would like to spread my legs out now and then, and you just can't do that in a mummy bag. I was going to place a couple of those warmers in my socks, and once in the bag, I just couldn't get my arms down there to do it.
Maybe try renting a few different style bags from REI and giving them a whirl before you make your decision, and shell out a lot of cash.