Success! I've got the 220.127.116.11 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 18.104.22.168 fix tonight.