Help! Need advice!

Its been a rough few days at our house.
Here's the story (sorry it's a little long) 3 weeks ago I added 2 pullets (a Houdan and a OEG mutt) to our flock, and was introducing them (slowly) to our flock of 3 LF and a Serama (all are 11 months old).
There was some pecking, naturally, but they all seemed to be getting along pretty well, I'd been letting them free range together. Then a few days ago (after one night in the coop together) our Houdan (Mabel) got a fairly large wound on the back of her head, I presume from our RIR. Thankfully my husband is a veterinarian and the wound is healing nicely. It was really upsetting to see that one of our chickens had wounded another of our chickens. I hate to think they aren't all happy to hang out together!
Worst yet, however, is that today our OEG bantam (Penny) is missing. Gone. No trace of her anywhere. No feathers anywhere. I'm so sad! It breaks my heart, she was such a sweet little thing and although we'd only had her a short time, I was really attached. I made flyers with her picture on it just in case someone carried her away but didn't kill her. Unlikely, I know, but I feel better having gotten the word out.
Now I'm unsure what to do with Mabel. If she was hurt by one of the others while Penny was in the coop with her I'm terrified what might happen if she's in there with the flock without her buddy. She hasn't rrosted with the others yet and had been sitting on the far side of the cabin, on the opposite side from the food and water.
Since she'd been hurt I'd been cooping Penny and her up in the garage with heat support in a big dog crate each night. I figured I'd keep them in there until her neck was completely healed and then start integrating them back in to the coop at night. Her neck looks good, it'll probably be all healed in a few days.
I need advice --
Should I keep her separated at night so as to keep her safe for a while longer or put her in with the other girls so she's not lonely?
Since she's new to the flock, should I get another or two pullets to keep them all separated for a few weeks and reintroduce them all together?
When they are free ranging during the day I don't see any violence in the pecking order, can I assume with now it's been established (with the big wound)?
Thanks for any and all advice.