I completed this mission on my first try because I hadn't leveled up Godric at all before. Now I'm wondering, if I replay the human Campaigns starting from the mission where Godric first appears and level him up properly, will his progress carry over to the final campaign?

It will carry over... but I leveled him up to the 24th(!) level in the human campaign, and in the necromancer campaign he was given to me "under command" at level 10... but on the other hand, with all the skills he had at level 24!:eek: