Fourth reinstall in as many days and I'm figuring the addons error is specific to some older custom-made missions.
The scripting files issue was me being a total [politically incorrect term!]. I forgot that when saving a new version of a mission in the editor, you have to manually copy all the associated scripts into the new folder. Doh!
Time to move on with 1.85...forward, forward...left, forward, right, forward, forward...