Can you not just create another account with a different email address? Regardless, hope you get it sorted soon.
I use Steam for all my games, but it was kinda annoying that the shortcut that RDR2 creates on install doesn't work... so I couldn't launch it directly from Steam. So I've had it set to launch the R* launcher, so I then click 'Play', but it bugged me that the process wasn't automatic. I put it down to the price of not waiting for the Steam release.
Tonight I discovered that if you create another shortcut from the RDR2.exe file, it will launch the R* launcher and then run the game in one action. I'm sure I tried that when I installed it last month, but it didn't work. It does now. Set that up as a non-Steam shortcut, and bingo - RDR2 now launches directly from Steam for a non-Steam version. Overlay works, screenshots work (no more being forced to upload everything to Social Club), although there are clashes with the Social Club, they're easily bypassed.