Good article and had basically come to a lot of the same conclusions mention here.
I had another similar issue which i though was worth sharing as it took a couple of days to sort out. As part of a move to 64 bit we installed VS 2012 alongside VS 2010 for a peek but decided to stay with VS 2010. I therefore uninstalled VS2012 and this is where my problems began. When i opened forms in VS 2010 (especially inherited forms) they would usually render in the designer fine. However, any subsequent close and reopen would cause a load of errors in the designer referring to “The variable ‘X’ is either undeclared or was never assigned”. Interesting enough was the ‘X’ usually related to properties of Infragistics controls that are used on our base form..
The issue turned out to be .net 4.5 that was installed as part of VS2012 but was not removed as part of the uninstall. I assumed VS2010 was still using it (as i could not see .net 4 under installed programs) so I decided to uninstall .net 4.5. At this stage (after uninstall) if you try to start VS2010 you will get “unexpected error” at the visual studio splash screen. Next step was to repair VS2010 which in turn installed .net 4. Once VS2010 was restarted the issues with the designer disappeared.