E39 perished windscreen rubber

Not on the E39 trims.

To be fitted properly, yes.

The guy (and others) manages to swap in with the glass in place although he does use a soap solution to ease the trim in. Inside the trim is butyl which helps grip the trim onto the glass. Butyl and soap don't mix.

It's clearly possible and many have managed to do it successfully. However, the design of the fitment is press it onto the glass edge before installing the windscreen. No washing up liquid is needed but a warmer trim does respond better to a cold one.

In three decades of fitting windscreens, the only application(s) where you would fit the trim after the glass going in is on a 'christmas tree' push-in fit on a Nissan or Toyota because they're designed that way (for the PUR to also adhere to the trim). I have never thought to do that on any BMW because it's a very different design and the butyl is doing the job the PUR does on the Japanese cars.

Good luck if you are thinking of trying it. Looks a long process and I'm quietly confident of not only doing it quicker by removing the glass, but overall, a better finish with no knife marks in the trim or car.

Out of curiousity if the rubber is perished what damage is done if it is not replaced, will it leak in water somewhere ?

That's fair enough and in the spirit of a DIY-ing car enthusiast.

It should be noted that not everyone will have success like many have had. The issue then becomes being forced into calling someone out when you're established in the task after taking the old trim out.