Coving over wallpaper?

Yes it would cause a problem. The coving ideally needs to be fitted first, applied to the wall and ceiling with the normal adhesive. The wallpaper then applied and to the bottom edge of the coving and cut accordingly to give a neat finish. Coving over the wallpaper will look odd and there will be future issues with the adhesive being applied to wallpaper rather than plaster.

I would check with your fitter before hand. He/she might be happy to cove over the papep. With the right adhesive it might be ok and he can caulk the join which will look neater. Other wise you might need to remove papera long the top which might be tricky.

Ideally you should not fix coving over wall paper as it could cause issues at a later date. worst case you could fix the coving over the paper but it could be a rough edge depending on the tye of wallpaper

It will not be an issue, you could put on the coving with some strong adhesive, as long as the coving is installed correctly, I can see it causing any issues at all