Are you wondering whether the Mercs would leave in the middle of the double quest? I would assume they would not, but they may ask for the additional gold to be deducted at the very end for a second quest. If you've won anyway and getting your reward, it's just a matter of settling accounts.
I like the advanced analysis done to gauge difficulty. I don't do all that but I do tally up the monster body points to get a general idea (or even more, give each monster an overall rating based on adding up their body, attack and defense... then get that total) and note the number of orange squares. Frozen Horror starts with the assumption that you'll be doing 7 quests in a row already fully equipped/maxed out from the Game System. Mercs would seem a necessity to supplement the four heroes. The issues with buying potions and getting mercs between quests here is no different than other quest packs. I wouldn't prohibit it, but come up with a clever excuse if the players demand one. "Do you realize how much of this dang mountain I had to climb before setting off the charge to blast my way in? This better be worth it! Also, these magic bottles will be charged to your account."
Don't let them ask why Mentor doesn't just transport a massive army inside the Frozen headquarters, or teleport all the bad guys inside the Imperial prison or straight to the bottom of the ocean. (Because Zargon is blocking my power; there are only small holes in the field I can get through; Loretome forbids me to interfere more than this; the ancient prophecies say this is the clearest pathway to success; opening these small portals weakens me enough I dare not risk more; honor demands we obey the regulations / imagine Zargon doing the same to us, it's mutually assured destruction!; have you seen the cost tables for teleportation these days? etc.).