Harald
I flew my Northstars (3 of them over many years) ONLY from water.
I built a derivative design that I call the Lochstar (one i.c. another electric) and it is flown only from water, except for one day in January 2010 off snow which worked very well.

When I do the elevon mix I configure the model as a delta to get the elevons.
Then I put the elevator servo on a spare channel and use a free mixer to mix elevator function into it.