Your issues are due to the fact that any file committed to the perforce repo is automatically set to “read only”, which means you probably committed your intermediate folder. You should remove it from the repo if it is. A quick hack would be to right click your intermediate folder and uncheck read only.
Well you can commit that, but if you do make sure you set the files inside to be ‘Always Writeable’ in Perforce, that way programmers can always build without warnings. You can then ‘check out’ binaries and commit them when safe, so that artists / non-programmers can use them.
Commiting .sln files / Visual Studio files makes things… messy. Just have everybody regenerate project files locally if they need them.