Efficiently moving things around in zig-zag Graphics buffer visualized w/ 24/32 bpp displays / neopixel strings

A) Is there reason not to group fontAbc and modMmmN

I'm not sure I understand?

B) What is STM32F4's alignment in structs?

4 byte normally I think, but the PACKED_FLAGS after it means it's 1 byte aligned

I assume SAVE_ON_FLASH flag is at runtime accessible somewhere...

I'm afraid not - all you could do is check for some function (drawCircle?) that doesn't exist with SAVE_ON_FLASH. These things were never really meant to be accessed from JS-land so there hasn't been any thought put into making it easy