What happens in 'assembly purgatory'?

Answer 1

Temp and tmp are temporary folders used during GAC Assembly installation and uninstallation. Tmp is used for installation, and Temp is used for uninstallation. Source: GAC Temp and Tmp

.NET Runtime Optimization Service (mscorsvw.exe) is probably probing files once in a while to check if they are closed so it can [un]install pending assemblies.

I may be wrong, but as far as I know, there is no WinAPI which would notify you that certain file has been closed/unlocked. You have to monitor them by yourself. But Windows itself must have such a list somewhere in memory (at least it has a list of open handles), so it just knows that the file is closed, because the handle representing the file is released.