dcghost,
And another problem. When i have two windows in the same browser. One window has CP with current site A, and autorized as member who has access to A,B,C. Second window has site C with link edit_this to the entry. The link will not work because session keep site A as current. Site A does not have the entry of site C.
This is default behaviour. The same if I was using MSM and just logged in normally to CP without any masking URLs and switched sites by opening second site in another browser tab. The first tab now needs to be refreshed as the session is for the second site.
So what you are trying to achieve here with your client’s workflow is not going to work. Is doing away the Edit_this idea and just having them login to the CP on its own and switching and editing content between sites an option?