My recommendation - keep the unkillable poop monster. My players (a party of 9th level dwarves of various classes) had an awful time with it and loved every second. Depends on your players, I guess.
Assuming that the PCs are going in through the Mausoleum, there are a few issues that you need to keep in mind regarding their power level. The upper levels are scaled at around CR 5 or 6, so you might find that the dreaded dungeon lacks a bit of whack in the opening areas.
With the power level of your group, you should give them a good reason to visit the Tomb of Saracek. It's pretty much the only thing on the upper two or three levels that can give them a serious challenge (unless you really hammer on them with wandering monsters, that is). Maybe they need to retrieve something from there (IMC, I had the dwarves assigned a number of objectives, one of which was to retrieve Saracek's crown for his descendants, who were wealthy sponsors of the dwarves' temple).
You should also try to get them to tackle the upper temple of Orcus - it should provide them with a nice, solid battle. A visit to the Oracle also gives you an opportunity to put Spiegel's advice to the test

.
Beyond that (assuming that you have enough time to get deeper into the dungeon), you are in good shape for letting them explore or adventure as they see fit. There are plenty of opportunities for them to bite off more than they can chew - which is what a visit to the Dungeon of Graves is all about, after all.
Or you could just convince them to go down the well...
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