I am desperately clawing at this file that suddenly stopped letting me select the bones while weight painting and now only selects the vertices on the mesh. I can not change any weight value on the model. I really don't know how to be any more descriptive than "it's only selecting mesh vertices in weight paint mode" because I had just made some weight edits a few minutes before and changed no settings, and now it's refusing to let me select any bones.
In order to select bones/vertex groups with Ctrl click while in weight paint mode, you have to select the rig in object mode, then shift select the mesh, then switch to weight paint mode.
If you can select only vertices and the paint isn't working, probably you have activated one of theese little buttons, which limit the action of the brush to selected vertices/faces only.