[StickMon]

When I got to the point where you are now, I folded it in over the ring and clamped it with binder clips all around.
Then I removed one section of binder clips at a time, about a quarter of them, trimmed the excess, and put the clamps back on.
After working all the way around, had it trimmed nice and even.
Then I removed one section of binder clips at a time, applied glue, and put the clamps back on.
I used Loctite GO-2 glue, but just about anything will work. Use it sparingly.
Once you put the ring back in the trim, the boot's clamped in there and isn't going anywhere, so it doesn't need much glue.

As for removing the Z4 boot, yea, it's more melted on than it is glued, so I had to pretty much cut it off and sand down what was left on the ring.

Your next challenge will be getting it back into the trim. The M5 leather is thicker, so it's a tight fit.
You of course do this with the trim out of the car.
Hold the trim upside down and carefully work the boot ring up into it a little at a time.
You may find you have to do a little extra trimming around the 2 stitched seams.