For the transform widget it's not possible actually, I think it's planned for version 3.0 but not sure.
But it's still possible to expose a simple 2D coordinate like in this pyplug : https://github.com/NatronVFX/natron-plugins/tree/master/Relight/ReShade
The setup is a bit tricky ATM, here is the doc :
It's been a long time I had this done, IIRC you have to use the script/text editor , access to the property you want to expose and set setUsePointInteract to True .
Once it's done it will work and you don't have to re-do it anymore.
I hope that will help, tell me if you want me to look a bit further into it.
About your second point, I'm not sure I get the idea, but I think you shoudn't have nodes inside the group that points to nodes outside the group, the group should be independent.