I took some time last night to complete the board's design. After a last-ditch effort to see if there was some way to include I2C on this board (short answer: no flippin' way), I moved things around so the mike output could go to any of A1-A5 with a wire jumper.
In a satisfying twist, removing the trace from the mike output to A5 allowed me to neaten up the other traces in that part of the board considerably.
I've also checked the measurements for the headers and mounting holes to make sure they match up with the Feather standard. They didn't, but they do now.
Overall, I feel pretty satisfied with how this has turned out so far. I may tweak the design a little, mostly depending on any feedback I get, but I think it looks pretty good from here.
It'll be at least January before I can start actually getting the hardware to test this out, which I'll do depending on (among other things) what happens with the Take Flight With Feather contest.