Fix For Elevon Routines
Found a fix for the Elevon problems (fixes them all, I think).
This is slightly complicated, but bear with me. In the end, you get everything working plus a bonus feature at the cost of only one mix.
First, plug your right servo into the aileron plug and your left servo into Aux 1 (the Flap plug). I've only tried this on one plane, so I don't know if those might be reversed in some setups.
Then set your Wing Type to Dual Ailerons. Now check your aileron control -- it should work normally for an elevon plane (elevator is not yet working). If it doesn't, you'll probably end up having to reverse your plugs to make things work right.
Next, go to Mix 1 and set Elev -> Flap with both rates (D & U) at 100%. In my case that was -100%, but you may have to use +100 percent in your setup. Check function; at this point it should look like an Elevon plane with a boatload of travel. Don't fret about that -- the extra travel will "fix" your trim buttons! And don't forget to set "Trim" to ACT in the lower right corner of this screen.
Now, if you want differential it should work perfectly. If it goes in the wrong direction, you'll have to back up and reverse the plugs (move the servo in Ail to Aux and vice-versa) and redo the other settings to reverse them. Ultimately it means you'll have all the function back at the cost of a single mix.
But you get some payback for that mix: your Flap switch will now work! Probably the best use for it on an elevon plane is adjusting the elevons to max glide duration in case you want to do some thermals and don't want to try and see what the little bugger is doing at 400 feet...
Undoubtedly your elevon travel will now be too great. If you go to Dual Rates and reduce your top rate down at least to 80% of full, your trim buttons will now be completely safe to use -- the endpoint travel will not be greater than your allowed travel, so the weird "button effects" go away.
As far as I can tell, this is a complete fix. Would be delighted to hear if anyone hits a snag... also this leaves the option of another mix open (if you're using rudder and want a mix, or wish to do a throttle -> "flap" mix to get the torque roll out, for example).