Thanks for all the advice, all.
The more I think about it, the more I'm sure I want a mummy. I considered a brain-in-a-jar briefly (fits the theme, actually), but I really like the idea of the mummy. I've decided to tone it down severely (call it a "lesser mummy"). Shave off a few hit dice and tone the save DCs down a lot, change the paralysis to shaken, etc.
I've also got a couple other things to mitigate the lethality of the encounters... Early on in day 1, they'll meet and save an otherwise-useless bard (dying in the desert, being circled by vultures...) who will have a wand of CLW (I actually rolled up 2 wands of CLW with 10 charges each in the total treasure for this adventure, so I'll give him a rod with 20 charges). So even if noone's playing a healer, they'll at least have a good amount of healing to get them through these encounters.
At the end of the second day (after the scorpions), they're going to stumble upon some leper-like gnoll pilgrims, led by a blind seer, who will totally heal them including poison and ability damage. Will also give them a clue about the mummy, and a cure for mummy rot (the gnolls essentially suffer from an arcane version of radiation poisoning, and will be wrapped in bandages, so it's a nice thematic foreshadowing, I think).
Question to those who say that there's too man encounters on day 1... I thought that it was intended that there be 4 encounters a day?