Hi Gena,
what you are talking about? We have only one branch "stable". Not sure waht you are talking about when you say tons of branches we have 1.
"svn checkout" "svn commit" are you talking about using command line!? No way in hell even considering that. People can clone and checkout with svn as well. If you need it.
Flow of contribution... 99% of patches require not quick review but rewrite, thus they are ignored, or delayed. When authors asked to fix it, or at least invest more time into understanding to the code the patching, they say that they do not have time for that, they expect some kind of "core developer" to fix it for them.
I do belive SVN provides enough configuration mangemnent for current developemnt team - 1 developer attempting to maintain most of plugins. git transition not going to fix it, just will eat pressious time from developing Miranda to playing with new configuration mangement system, redoing build system, update system, and trying to find GUI tools to avoid using all these command line nightmare you are proposing.
Are you interested in working on Miranda code or switch SCM systems back and forward.