Well, I believe that global menu support is done not only for mate-tweak, but in general so that for example vala-panel-appmenu-mate can work with the mate-panel (which can be added even without the mate-tweak). Mate-tweak uses some layouts like mutiny, which add the global menu applet and so it needs to make sure that it works properly.
The problem in my opinion, is that the documentation is really missing. Maintainers/packagers don’t have any info on what works and what not. All the info that I’ve gathered is by reading the mate-tweak code and I’ve done it without even knowing python!?!?! (I needed to read the /usr/bin/mate-wm script and some code which, thankfully, had enough comments in order to understand how to fix the original problem with the compositor).
I believe that the best we can do is to treat the latest mate-tweak version as a development release. That means, that when mate-tweak 18.04 is released (which will target the next Ubuntu MATE) then we should NOT update it. We should wait for Ubuntu MATE 18.04 to be released first and hope that any needed patches will have already be included in the official MATE projects (mate-panel, mate-settings-daemon, etc…). That makes sense because in the last 2 years I’ve seen that the next MATE release is always done just before the next Ubuntu MATE release.