I am sorry to hear about your crayfish escaping. Have you found him yet?
I think the reason mine doesn't get out of my 55 gallon tank is because it is deep and it was prepared for climbing critters. Before I got the crayfish, I had fiddler and red claw crabs (one of each). I would often find them hanging from the back of the fish tank or in my whisper filter. I caught one running across the living room. So I placed the air tub I had through a hole in the corner of the back cover so if they climb up it they couldn't go anywhere. Then I shortened the intake tube for the filter and moved my high rock formations way from the filters intake tube.
Do you know how the crayfish got out? I would think you should be able to fix your 5 gallon tank so that a crayfish couldn't escape. Then you could get another one.
You can put other fish with a betta. I would recommend only a few and preferable only those classified as community and no tiger barbs or other fish known to be fin nippers.
I would be more specific but I've had bad luck with Bettas and I think it is because I always had at least two tiger barbs in the tank. Since I like tiger barbs, I've opted to not get Bettas for now.