Sorry, but I am not a SVN user, apart from fetching C::B from SF. As such some of the SVN info below I only just looked up.
Thank you for a lot of info, I am also not an SVN user

. Let me clear up some points, though. Note that my ambition is not to reform the Code::Blocks main repository, I just want a convenient mirror on Github.
Ideally, if you 'git clone' from that mirror, the result should be the same as for 'svn checkout' from the original SVN repo. It turns out that this is not 100% the case now because the contrib plugins
FortranProject and
PythonPlugins are missing. So why are they missing?
It turns out that in SVN you have something called
svn externals which is kind of similar to
git submodules, but not quite. SVN Externals are references to separate SNV repos, but svn externals and git submodules are sufficiently different that git-svn doesn't support externals. There are
many discussions like this on the net, but no really obvious solutions for a case like this, or at least I haven't found one.
For now, developers who want to build C::B with plugins FortranProject and/or PythonPlugins should use the svn repo directly. I can't really see how a github mirror can accomodate such externals without messing with the commit history. At least it would require some research to come up with a solution, and personally I don't use those plugins.
If the repositories all migrated permanently to git, then FortranProject and PythonPlugins could become git submodules.
Your points:
1. See above
2. See above
3. Yes you can build a Fortran project without the
FortranProject plugin, but maybe miss some features like debugging.
4. Those arrows indicate
svn externals To obtain a git mirror with all external plugins require solving the svn externals => git incompatibility issue.