Aha! Seeing the name "vinegar flies" for the species let me figure out the smell coming from the tree. It smelled like vinegar. I don't cook with vinegar so I didn't recognize it by name at first, merely as a vaguely familiar odor.

Fermentation explains the odor I couldn't identify.

If I get the chance to pick a shot, what are the key features to photograph to identify drosophila to species or species group?

Like the one you sharpened up, the mesonotum, dorsal view, showing number of rows of hairs, orientation and presence of various bristles and colors patterns.

The banding patterns on the dorsal abdomen are also great to have: if you can tell whether the bands are interrupted along the mid-line, and whether they are any obvious spotting patterns.

Foreleg to see sex combs.

Wing coloration: clear versus clouding at cross veins or tips.

I've been looking for a slime flux for years. OK, not really searching a forest for one, but being on the look-out, thinking I had to run into one eventually. See all the flies you found inspires me to keep at it.