Nice Write-Up - If you do any google search for this type of issue, your post based on 'attach bmw rain sensor' comes up at least as high as number three. Anyway, followed your suggestions with the exception of attaching the prism to the windshield with the silicone first, then attaching the sensor to the prism. I just attached the prism to the sensor in the first place, put some silicone only on the prism and attached it right to the windshield.
The only issue with doing this method is the windshield has a nice little cutaway for the prism to line it up and when you attach the prism and sensor together, you can't get lined up perfect. But, the silicone allows you to adjust it until it lines up. Give it 24 hours to cure and you're good to go.
Thanks again Wadwyer!