Mixing lanes: Jim's been big on the Tesseract TexMix series, and I've come to appreciate the simplicity and size aspects they've got. Plus, you can build onto a TexMix if/when necessary...with quad mono or stereo strip modules, and you also get a really nice AUX send/return implementation.
The only other thing that jumps out at me as far as changes to the present build is that you might want to move the buffered mult and precision adder in row #2 to the left end of the row. That way, they're a little more convenient (you don't have to dive into a patchcord jungle) and the cables to the oscillators can be dressed over the top of the unit if needed.
E370: Don't take it out! If it fits as well as it does here, then that's a sign you're doing something right.
Switching/logic: BassBow II might be able to do that, but it might be even more interesting to use a bunch of branched 3 or 4-step sequential switches (like Doepfer's A-151), and then drive them with various clocks. With that, you can whip up some really complicated sequencer line behavior. Screams for Boolean logic!
Ya done good, Nick!