I am even more impressed on hearing the Felixstowe was the first biplane that you rigged. The Staaken looks an interesting build, I seem to remember Ray Rimell wrote an article in Scale Models (I think it was around 1980) about scratch building one,it was enough to bring on the vapours.
As far as Roden decals are concerned,I built a Roden Gladiator not long after the kit was first released,the decals were terrible they wouldn't release from the backing paper,were brittle and wouldn't settle down, they were like cardboard,decal solutions had no effect,I ended up using aftermarket decals. Having spoken to other modelers about them I would say this appears the general experiance.
I admit I have virtually no knowledge of WWI aircraft but are there any aftermarket lozenge sheets available that you could use in place of the kit decals? Maybe because of the size of the Staaken none are available. Where any Staaken's finished in a plain scheme which you could use as a fall back if the kit decals are a problem?
As far as decal life is concerned I think it depends on the printer and how the decals have been stored. Last year I built a BPF Corsair and used some decals from a Modeldecal sheet which would have been around twenty five years old and had no problems with them. I think it can be the luck of the draw as kit manufactures seem over time to change decal suppliers and hence the quality of their decals change. Therefore different production batches of the same kit may have decals from different suppliers and handle in different ways.
All the best,
Malcolm