Is there a way to detect programatically if MapPoint is installed?

The best way is probably to check the registry. MapPoint sets some registry items at the HK_CURRENT_USER\Software\Microsoft\MapPoint level.
The different versions have registry folders here, and there might also be one called "Addins" that is used to tell MapPoint of the existence of Addins.

Richard

PS: Glad you've got the fields working!