I agree with all choices; and with reasons 1 - 4.
5. French forces is not a plural countable noun. But let's assume it read "French soldiers" (which is a plural countable noun). The answer would be the same (zero article). The reason must be different.
6. "French soil" means "French territory" (it's a metonomy). And thus it means "France", which doesn't take an article.
I don't know the system you're using, so I can't give reasons for why 5, 6 are wrong. But your answers are right; and you reasons seem wrong.
Student or Learner