More info after testing with new save:
2. Tried crafting with 2 furnaces, 4 plates each for a total of 8. After taking 4 + 3 plates, the info on the left showed only 1 plate picked up. After taking the last one from the second furnace, it changed to show 8 plates.
4. Achievements for 1 and 5 tech upgrades do not complete even in a completely new save, even after it's finished. Single mission completed fine, so the achievements work in general.
Saves will be left as they are, in case they will be needed later.