Did the old screen trim on the red 3.2, although 'manky', sit ok? If so, it's obviously the replacement aftermarket goalpost trim that isn't of the correct profile to sit flush. From experience, the channel in the plastic retainer is likely to have collected a fair degree of silt over the years which needs cleaning out. I would then fully clean the 'goalpost' trim, but I have found it is a lot easier to fit it snugly by applying a thin smear of red rubber grease on the tongue. This prevents any damage to it when guiding the tongue into the retainer, and helps it into place.
If the trim didn't fit snugly before, then you only real alternative is a new trim from VX - they are available in Germany although Autovaux seem to be able to supply and deliver the genuine article cheaper than VX.
You can actually replace the retainers with care whilst the original screen is in situ as long as the goo used hasn't sealed it to the screen itself. But you need patience and a lot of care to do it without damage. The upper retainers are unfortunately no longer available from VX....
When refitting the retainers, I have used a few blobs of black Adiseal to keep it in situ. It's a bit more flexible than Sikaflex which I like but sticks like the proverbial...
That is categorically untrue.
I learnt this the hard way fifteen years ago. BWTFDIK
Obviously not how to get the retainers off a screen in situ
You can do it without damage because I have done it. Like I said, it takes a degree of patience and care, but is doable, as long as the goo has not bonded the retainer to the screen.
I do you recollect you categorically saying earlier in the year, when I asked a question about the lower retainer being partially loose, that it could not be removed with the screen in situ without damaging it, but as I have said (a few times now) with care, I removed it, cleaned and replaced it, and it is still on the car today snugly holding the lower moulding against the scuttle and screen.
Should I say BWTFDIK? No, I wouldn't. But what I have outlined, to possibly help others, is from actually having done it....
If the mouldings are not deformed or damaged, and the retainers are not damaged, there is no need to use sealant to hold the mouldings in place in the top corners.