A partial solution that seems to work (mostly) is to use 3 mixes -
Mix 1 is always on and acts to CENTER CH3 (Master channel: Steering, Slave Channel: CH3, Low side mix: 0, High side mix: 0, Offset: +100).
Mix 2 is assigned to SW2 (toggle) and provides "reverse phase" (4 WS primary mode) (Master channel: Steering, Slave Channel: Ch3, low side mix: -100, high side mix: -100, Offset: 0)
Mix 3 is assigned to SW3 (toggle) and provides "same phase" (crab steer) (Master Channel: Steering, Slave Channel: Ch3, low side mix: 100, high side mix: 100, offset: 0)
I'm new to mixes, but apparently Mix1 + Mix2 yields 4WS, while Mix1+Mix3 yields Crab Steer, and Mix 1 alone (or Mix1+Mix2+Mix3? ) yields Front Steer... The driver must remember to toggle Mix 2 off before toggling Mix 3 on... etc...
could use a few more Switches and fewer Trims... SW1 is assigned to CH4 for 2-speed transmission low/high shift.
Is there a "bug reporting" email/site for the firmware...?