There is no set timeline for merging the 3.0 branch into the master branch. We want to make sure everything is working properly before moving forward. We do have plans to support reading 2.x earth files and to continue support TileSource plugins; these things are still in the works.
The 2.10 branch will remain for those who wish to stick with the existing API. Bug fixes and minor upgrades will occur on both versions as long as it's feasible.
All questions are welcome and we appreciate any constructive feedback!
Thank you and enjoy.
I don't know the specifics but we were told that there was a problem with using OSG 3.7 with osgEarth a few months back due to a problem with the graphics pipeline or something like that. It was recommended that we not use any version later that OSG 3.6.2 which has been working fine for us.