1.
That repository (containing the full history of EPICS) is quite large.
In the current setup, you are able to have a single clone of that large repository, and check out the three branches into three different places.
If you separate out the modules into separate repositories, you will always have three copies of the large repository.
2.
We are heavily using the very powerful issue tracker on Launchpad. (E.g. one single issue being tracked separately in several EPICS release series.)
The GitHub issue tracker on the other hand is quite simple and limiting. (E.g. can't move an issue to a different repo.)
We don't see a way to losslessly export the issues from Launchpad and import them to GitHub.
Once there are solutions to these issues, we might want to reconsider the general repository layout.
Note that we are discussing to bring the three original Base parts back to be on a single branch to reduce complexity and avoid the unusual setup that seems to cause a lot of confusion.
Cheers,
~Ralph