Thanks to Pierre Martineau, who writes: "I think you'd like to know that the "iridescent fly" (images 3-7...) is actually a bee; I believe it is a species of Ceratina, a genus of (cute) small carpenter bees." I should have known: no halteres.

And Pierre adds later:
"Bravo Dan for the "no halteres" comment! makes me feel quite proud of my student :-) Also, carefully looking at your pictures at the highest resolution might reveal one on which you would be able to tell (I was not) that there are in fact 2 wings folded on top of each other; this would rule out a fly.

"Another striking (yet more difficult to formalize precisely) bee character on your pictures are the 'elbowed', ant-like antennae. AFAIK, no fly has such antennal structure"