flight mode
While your "latest rumours from the Swiss side" may or may not prove to have substance, Swiss operated LHR-SIR last winter and will do so again in February 2018 using Helvetic ERJ190 aircraft but you didn't mention this.
It is a category C airfield but as it would be Powdair's primary airport I don't see crew certification/validation being a major problem.
The Powdair website now says tickets will go on sale on 6th June and their comment ".... wait until you see why (there has been a delay in this happening)" doesn't suggest the sort of problems you mentioned are the reason for the delay.