I'll break this down to try and answer everything completely (I feel like yes or no answers may cloud the issue a little).
Lisa:
I know one of our casters 9Gustav from memory) is really tired and would walk very slowly but under the effect of a magical Fly would he not fly at a faster rate?
Absolutely yes. The spell flight is giving a
lot of benefits to both Gustav and Rolf right now. Despite the fact that Rolf is 1/2 move and dodge, he still flies at 6 yards per second (and I see no reason that he can't fly backwards at the same rate). Gustav flies at 8 yards per second with no changes to his normal move or dodge (differences in speed due to encumbrance level, not fatigue).
Lisa:
Also, if the two flyers picked up Terri, would they have a decent (Flying) movement rate?
Probably. I would have to do the math involved, but I would suspect that is the case.
There are lots of factors to consider though -
First and foremost of which is that Rolf is exhausted due to fatigue, and his ST is halved. While I'm ruling that it doesn't change his encumbrance level for things that he is already carrying, it definitely
will affect his ability to try and pick up and hold a person. Again, more math for basic lift, etc. that I just don't plan on going through unless someone says they are
doing it.
Second of which is time. Both of the men could fly backwards out of the cave without too much trouble, but in order to pick up Terri, they would need to 1. turn around and 2. Grapple Terri before flying back down. Running away is always dicey in hand to hand combat and the extra 1 second and -2/-4 to defense may be more than enough to dissuade either or both of them.
Not to say that I think this is a bad idea - I just want to convey that you have run out of easy choices. There is risk/reward everywhere and it is up to you guys as players to determine if it's worth it...
... unless you fail a sense of duty roll (if you have it). Then you
have to help Terri.