You can look up the MOT testers manual on line . I would think that possibly insecure inner wing linings could potentially fail as insecure body components under section 6.2.1
https://www.gov.uk/guidance/mot-inspection-manual-for-private-passenger-and-light-commercial-vehicles/6-body-structure-and-attachments#section-6-2-1Thats not to say that yours is bad enough to fail. Difficult to say without being there. Maybe not . Its the subjective opinion of the MOT tester of whether it could potentially be dangerous , now or soon. Taping with gaffer tape may solve a sharp edges problem that could injure pedestrians.
But could the panel move by enough, or become insecure in the future ,that it could potentially catch on the moving tyre and fold up within the wing and jam your steering on the move.? Or fly off on the motorway? I've seen cars with insecure platic parts that really catch the wind at motorway speeds and flap around alarmingly.
. I personally would want a car thats safe, not just one that that narrowly manages to get through the mot. Not always the same thing. I would make the effort to get the panels secured properly. Ideally with the original type plastic plugs and fixings. ,or universal equivalents. Or with a bit of ingenuity and maybe drilling a few small holes here or there secure it with self tapping screws, washers, small nuts and bolts ,etc.
Sometimes a small prang to front skirts or a wing can misalign panels and their fixing holes. You might be able to ease them back into alignment, possibly even using a strategically placed car jack to put some some gentle pushing force on the panel. Or by drilling new holes. I have used both methods in the past.
I also agree that having tyres with significantly different tread depth or tread pattern on the same axle is not recommended.