You didnt seem to have tried the solutions you said.

I would still try the solution in this post you've mentioned Where they state:

"Right click the .sql file within Visual Studio, then click "open with", then click "add..." in "Open with" dialog. In the "add program" dialog type "explorer.exe" into the program name field and somehting into "firendly name" field e.g. "with explorer", then click ok. Then select "with exlorer" in "Open with" dialog and click "set as default". Now VS will open .sql files with explorer wich will in its turn call SSMS."

And also try the registry change

HKEY_CLASSES_ROOT\sqlwb.sql.9.0\Shell\Open\Command, replace /dde with %1

Also keep in mind if it's a bug, which sometimes happens, you'll get the fix in the next update, which will be pretty soon as they update ssms regularly. I remember a pretty problematic bug ssms had a couple of years ago, I think it was with the data import feature. We were forced to wait for the upcoming release.