1.
This might help with the default keycodes, specifically the _SFX layer. Maybe customize that keymap.c to suit your needs? (I suggest brackets.io if you're in need of a decent text editor)
2. settings / save configuration — This will save a .json file to your computer. next time you want to make changes, click upload at the initial screen.
disclaimer: Seems to be a slight bug with this feature, as i've had to upload the .json a couple times for it to load at all.
3 – May or may not be helpful, but I've been flashing an AMJ60 with
QMK Flasher, and all I've done is press reset on the PCB, then flash the hex file. Flashing the sentraq may be different, though.