As regards the caulk round the bottom edge - remove as much as you can before anyone sees it.
As regards the large hole, have a look round a builders merchant or large DIY shop in the uPVC cladding section and see what you can find. Something like a corner strip for fascias may do, it you cut it down/adapt it a bit.
If your not good with mastic. Masking tape either side with gap where you want mastic. Apply. Run finger down. Pull tape. Run finger down again.
As above find plastic trim end or something and adapt for that hole. Make cardboard template if required to get shape.
Don't use superglue as no give and it cracks plastic when it moves with heat. Use a polymer grab like CT1 or a Ob1 or cheaper version.
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