OK, so we need:
High/Low - Lock in place
Up/Down (Forward/Reserve) - return to centre
Turbo Shifter (High/Low with Turbo)
Turbo Shifter - return to centre.
4/5 way shifter
The Shifters w/ Turbo button can be handled with a control panel button instead, so you don't really need that.
High/Low - Lock in place
Up/Down (Forward/Reserve) - return to centre
Turbo Shifter (High/Low with Turbo)
Turbo Shifter - return to centre.
4/5 way shifter
A 4/5/6 way shifter is usually a lock in place shifter, right? So why not use gear 1 & 2 on the 4/5/6 way for your High/Low lock in place shifter?
High/Low - Lock in place[/s]
Up/Down (Forward/Reserve) - return to centre
Turbo Shifter (High/Low with Turbo)
Turbo Shifter - return to centre.
4/5 way shifter
Starting to look manageable now.. but can it be cut down to one? Maybe, IF somebody could hack a 4/5/6 way shifter the right way, it could work in lock-in-place mode AND in return-to-center mode. I'm not suggesting a hack that would change the mechanics of the shifter, what I'm thinking is to add a 2nd microswitch to at least the gear 1 & 2 positions. How to get 2 switches on those positions would depend on the specific shifter being hacked, but I'm sure something could be managed. Assuming you could manage to mount the extra switches, here's the magic part: Either find a momentary-contact type of switch or hook a standard switch to a circuit that would in turn close a contact for a half second or so. That gives you the electrical equivalent of a return-to-center shifter, even though the shifter does not in fact return.
Does that make sense? It does to me, but then I already know what I'm trying to say.
Anyway, if it could be pulled off, then you'd need:
High/Low - Lock in place[/s]
Up/Down (Forward/Reserve) - return to centre
Turbo Shifter (High/Low with Turbo)
Turbo Shifter - return to centre.
4/5 way shifter with momentary-contact hack
And that's all!
PS- Have I done this? Nope. I hope to, someday. For now i'm just spouting off my crazy ideas, so if it turns out I'm talking out my bung, sorry!