Sorry, didn't mean to sound flippant. Just that I've been there, bought the t-shirt. And there are so many of these squeaky door seal threads I'm probably repeating myself or telling you something you already know.
In my opinion it is bad design on BMWs part but it's not limited to BMW. My A3 had the same door/window frame set up as the E9x and that squeaked too. I'm convinced it's something to do with whatever products are used to clean the car and these contaminate the seals. I tried degreasing just like you as it made perfect sense to remove anything previously applied or used to clean the bodywork. The noise returned though after a week or so (warm weather makes it worse) and in the end it was only the genuine BMW stuff that solved it by applying to the felt areas only. I can now go 12 months with no noise.
If you do get the BMW product it's 83 19 9 408 523 Spezial-Gleitmittel fur Turdichtungen or Special Lubricant for Door Seals. It's also very sticky and I've just got it all over my keyboard. You're welcome to try it if you're ever in Sheffield. I've also sent this stuff to a member based in Malta and also answered a query about it from a US member the other day.