I did try a search on this but I didn't get much in way of helpful archived material. For the sake of argument, let's say I want to discharge Nalia from my party in order to pick up Valgyra (sp?) in order to do his quest. Now I have no plan on keeping him once it is done and lets say I want to get Nalia back. Is this doable?

1. Are there any characters that stay away if they are even once expelled? I had Cernd, expelled him, but he said he would wait for me at the Druid's Grove if I ever needed him again. So if I need him I at least confident I can get him back. Airie on the other hand seems a whole lot more fragile. I am not sure if her character could take the rejection, or even that diesel woman, Jehiera, for that matter.

2. Any other advice will be most welcome.

yes it is doable. For most NPC’s there is a dialogue option where they tell you where they can be found, Minsc for example goes and hangs out at the Copper Coronet and some NPC’s have an option where they will remain where you left them just waiting for you to get interested in them again. I forget exactly where Nalia goes but I do know you must have saved her family’s keep before a dialogue option comes up. How willing a NPC is to go and just wait for you depends on the status of their personal quests, your reputation and their moral. So yes some may be unhappy and leave forever.

One thing to remember is that if you expell a party member that you are in a romance in, the romance will be immediately terminated.

You will not be able to start the romance again short of cheating.