I think it is a larva, and that the "tubercle" is the anterior spiracle, though I too am somewhat uncertain. The best images I've found giving a clear comparison of Microdon larvae vs. puparia are on page 8 of this PDF for a paper on a Brazilian species.

I recently ran into two interesting 1981 papers on Microdon, which I added to the info page (see bottom of Print References there...there are links for online access). The second paper has keys for Nearctic adults and puparia. The first key break for puparia splits species among those with dorsal surface smooth vs. reticulate. There are 6 (or 7) "smooth" species and 10 "reticulate" species in that key. I'm guessing that dorsal reticulation on larvae generally persists in the puparia. Your larval images here, as well as all those in the thumbs below, have clear dorsal reticulation. (Note the last thumb shows two larvae and a puparium...we think!)



In fact, the only BugGuide post that I think may qualify as a "smooth" species is the one below, which is also a bonafide puparium.



But even there, I think I can make out a vague reticulation, which I'm imagining could be the vestige of what may have been a clearer reticulation on the larva.
I guess us BugGuiders will have to work on finding more "smooth-backed" Microdon larva/puparia :-)