Lots of ready made collars available to get a seal around any pipes.The joints can be tapped down for any loose areas on the roof or removed and reapplied with new seals .Lots of videos on YouTube showing simple roof repairs .
Appliance should be good for at least 6 years , go back to supplier and quoting consumer act 2015 tell them its not s viable repair , they should offer partial refund .
Just drill to depth of plasterboard, if you hit metal stud a plasterboard screw will self tap into it. If it's clear then use your plugs. I prefer a screw-in plasterboard plastic plug, as if it touches any cable it will push it to one side...
Nothing you mention is detectable by the human nose . Damp and dead vermin do smell , maybe one of those .From your description this is a stairs to loft not ground floor as loft is above?
Lots of online videos on how to repair holes in plasterboard .