How to prevent leaks in the cabover area?

Agreed on not using EB on side windows. Use butyl tape on those guys. Works great and it's designed for that purpose. I would for sure use it top side on the long roof seams. Put it under the side mouldings and with a long overlap onto the roof. At sale time, I would see that as excellent preventative maintenance. It is also easy to inspect. Dicor dries and has to be scraped back in order to re-apply. You don't reapply with eternabond. It actually does stretch/move with the body and self seals. Applying around corners does require some "work" to get things right, though.

I also think you should put Alex around the top and leading edges of your windows. That is where gravity and wind pressure try to push water in. A little extra protection to help the butyl tape is welcome in my book.

Jose