"My point is that with GPL, the implementation of the fixes/new features is right in front of you, you see it, yet you can't merge it into your GPL-incompatible codebase."

1) You almost certainly can as long as it's trivial, or you didn't look at the fix but did it independently based on a description of the bug. Legally, it all comes down to whether your work was a derived work or the original, not whether the result looks the same.

2) That's the idea anyway! Allowing someone to copy the work of others into a codebase that doesn't allow similar copying is exactly what the GNU GPL is designed to prevent. It's a risk you take if you decide to maintain a proprietary fork.