Success! I've got the 126.96.36.199 version of AdditionalMapTypes working. The seeder now seeds and the harvester harvests. The issue was in fact with the modDesc registrations for the new crop types. Changing fruitTypeGroups="grainHeader" to fruitTypeGroups="sowingmachine grainHeader directCutter" solved the problem.
I'm a bit worn out from work so I dont think I'll put much effort into the 188.8.131.52 fix tonight.