Being that mummy rot is a curse clasified as a supernatural disease it has a very specific method to recover. I would suggest that the amputation is an assisted Heal check in place of the farmer's Fort save to keep him from suffering Con and Cha damage for the day. However, you should make it clear to the players that the disease is still present and deadly. The cranberry farmer survived the attack so the players might realize how powerful a foe they will have to deal with (how about Knowledge-religion). I included it for metagaming purposes. As for the likelyhood of the farmer surviving the attack, it was feasible that a commoner that engaged in one round of combat with a 3.0 mummy then fled (and was not pursued) could survive. However, with a 3.5 mummy striking for a minimum of eleven points of damage farmer Burnean should have several levels of the commoner NPC class to see him alive through the slam attack. Speaking of Sethelen's 3.5 power increase, be aware that even if your players manage to lure the mummy through the weakened floor and ignite him with the oil Sethelen will survive and be more than they can hope to handle. The mummy's slam will almost certainly hit and kill anyone he attacks. Run the encounter with caution be sure to allow Knowledge (religion) checks to identify Sethelen as a mummy, perhaps have the mummy waste a round or two smashing parts of the library in his rage while potentially paralyzed PCs witness the powerful display.
I'm glad to see the adventure still gets use after the drastic mummy changes. Enjoy.
Sounds like you hit the perfect balance to create a very memorable encounter.
Thread Necromancy to the rescue!
I ran the adventure as-is (3.0 stats) but with 3.5 adventurers. There were 4 players: 2nd level barbarian, 2nd level dragon shaman, 2nd level rogue and 1st level bard/beguiler multi.
Each player was given a choice of either a magic weapon or magic armor, and (wisely, I thought) everyone took a magic weapon except the bard/beguiler.
I ran Seth with the stats as they were in the mag, and although I got a few good hits on them they lived quite easily. The battle was hairy for many rounds, with decent die rolls on both sides of the DM's screen.
Eventually, though, Seth's die ran cold and I started sucking eggs. With Seth's DR, the wounds inflicted by the group were minimal, but they "nickle-and-dimed" him down round after round, whereas when my dice ran poor he missed them repeatedly. The barbarian raged when her hit points got too low for comfort, and she easily dispatched Seth to his final rest.
They found Irewyn, brought her back to town disguised as a male elf (long story) and requested that Father Denethan accompany them back to their inn, where they revealed that Irewyn was still alive. We needed to wrap up, so no cool cliffhanger ending, but next week they intend to have a "town meeting" where the traitors will be exposed.
Good times, fun adventure, and not too hard but not too easy, either. Thanks for writing and publishing Bogged Down!