This one had a hefty, tough stem about 5-6 cm in diameter and large overlapping lobes with no classic fins underneath. Each lobe was fairly thick, ranging from maybe a centimeter at the edge to 3-4 centimeters at its base. Does this description match "Armillaria, probably mellea"?

Yep. If it was just one mushroom at one time, then it is not Armillaria mellea. That is what I get for trying to identify what looks like bear vomit. By the way, you seem to be investigating some pretty gross things. Dung, carcasses, rotting mushrooms. That is the mark of a dedicated naturalist!
Now, large, overlapping lobes with no gills? Perhaps it is a Polypore of some sort such as Grifola frondosa? Then again, I would say this is too far gone to be ID'ed. It would be interesting to note whether certain of your insects are collected only from certain species, however. The only distinct species associations I know of are horned fungus beetles on bracket fungi, and calliphorids, silphids, sarcophagids and such attracted to stinkhorns.

is to go where no man has gone before, even if it's elbow-deep in "bear vomit," seeking out new life forms. More accurately, I try to think of things that *something* will eat, and chances are there are beetles there, munching away. Also, the more disgusting the substance is, the less likely it is that some sissy will have already submitted a photo of it to bugguide.net. Keeping one step ahead of the sissies, that's *real* important to me, yuck! ;-)

At the moment, I've got some fermenting fish under a weighted-down wire basket to see what late-season visitors it might lure. Just a few yards away is a bountiful patch of large bright orange mushrooms that should lure *something* worth photographing when they rot. (I'll submit pix of the fungus when I post pix of beetles found on it.

I have a friend who sets pig carcasses out to gather flies and then measures maggot development rates within the putrescent body (I like that word: "putrescent"). Has a nice ring to it.
Anyway, I am not that big a sissy, just the smell of rotting flesh often makes me want to chunder. That and jellified rotting cups of mosquito pupae that I forgot to remove from my cages...Or a sausage skin filled with beef blood forgotten 4 days in a 28 C incubator...I suppose entomology IS gross... sometimes.

Armillaria would probably be made up of many individual mushrooms. By the way, I think the "fins" you're talking about are what are usually refered to as "gills."

I just forgot ;-)