I was surprised to see that this morning, all the larvae appeared to evacuate the galls. I'm not sure what the reason is but I've never had this happen with midges or wasps before.

is probably just that they're mature and ready to burrow and pupate. Ray might have a better sense of the ratio, but my sense is that the (vast?) majority of gall midges exit their galls as larvae and burrow to pupate, although there are common/large genera that pupate in their galls.

I think most of the larvae have exited. Do they complete their larval stage so quickly that this is no coincidence?

This is news to me. I have raised galls on Salvia in the past, and adults emerged from the leaves.

I don't recall ever having midge larvae exit galls prematurely (except simple, open, leaf fold type galls that were deteriorating and no longer hospitable), but if you saved a few in alcohol Ray could tell you whether or not they are mature (and if they are, could provide some sort of ID).

Interesting note about soil...