I'd take Reina back if he was available. He may have been out of form when we let him go but there were probably good reasons for that.
Migs seems a nice guy but don't think he has a strong enough personality. Grobelaar used to make loads of mistakes if I remember, but he seemed very strong mentally and always appeared to be confident. Reina is similar I think. I don't think Migs has been an improvement on Reina really.
I would sell Migs, get Reina in as no 1 and give Karius a whole season as no 2.
You've summed part it up nicely when you say Mignolet isn't really an improvement on Reina. And if you look at it the other way Reina isn't really an improvement on Mignolet.
Also, to everyone those goes on about Reina kicking it isn't very good compared to the top keepers around.