Could you run a 10 speed mech with an 11 speed shifter and 11 speed cassette. Or will the throw be all out. Last day riding in the alps and my mate has killed his rear mech, but I have a spare 10 speed . Will it work?

Theoretically no, but my guess is it’ll work in some of the gears, and be ghosting around the middle range of the cassette.

If you’re only riding the dh tracks you could maybe set it up so it’s optimum in a good cranking gear for the terrain, or even chainless.

As above… I have had to do this. Works fine on top 3 gears at back but couldn’t get rid of slight ghosting and hunting in the middle of cassette.

Try running the cable the other side of the cable retaining bolt hole, that gave me more useful range but did not fully eliminate the miss shifts.

Also depends how worn the rest of the drive train is! May be fine if it’s newer as everything wears it develops play and larger tolerances.

It will be better than the alternative (no riding). Swap the cable routing and trim for the middle of the cassette.