I'm not going to reply to any specific points, but I do want to comment that I've come to believe that e-mail based patch review is a problem. Unfortunately, I do not foresee the GCC maintainers moving away from it.
I believe that Diego tried setting up an alternative patch review system using Reitveld, but it did not catch on. For Go development I have been using Gerrit, an instance hosted at Google (https://go-review.googlesource.com/). It's just a much better way of doing patch review. You can do line by line comments, you can easily see the set of all patches and their state, you can assign the patch review to specific people, you can easily commit the patch. For Go we have trybots, which can be used to build the toolchain with the patch and run all the tests on a range of different architectures and operating systems. Gerrit is free software under the Apache license (https://www.gerritcodereview.com/). It's not perfect: in particular, there is no offline code review. But overall it's just much much better than what we do today in GCC. But the active maintainers are accustomed to the current system, and it would be very hard to change. Ian