Oh dear

Have you tried the old trick of getting in the car and getting someone to spray a hosepipe over it? that'll help you find where its leaking
my RSi has similar corrosion in that area, i know theyre not the same car but check your bulkhead for any signs of corrosion as it my be leaking in there

Do you have a sunroof? check the drain channels arent blocked, also check the condition of your door seals (use the hosepipe test)

What is the sunproofing made of? can it be removed? The soundproofing on my RS just acted like a big sponge and rotted my front floors pull it all out and get rid, it does more harm than good

If youre going to sandblast the underside i think you should strip it completely, you dont want to damage any vital components