"Jane wouldn't have found a job unless she had gone to London" is a natural-sounding sentence and has two different meanings, depending on whether Jane really did move to London or not:
(1) "Jane wouldn´t have found a job if she hadn't moved to London" (i.e. She found a job because she moved to London)
(2) "Jane would only have found a job by moving to London" (i.e. she didn't move to London so she remained jobless)
On the other hand, "The situation would have deteriorated unless I had arrived" is not natural-sounding and can only mean:
(3) "The situation would have deteriorated if I had not arrived"" (i.e. I arrived so the situation didn't deteriorate")
My questions are these:
Is "unless" only correct in third conditionals where the sentence remains true whether the condition occured or not - i.e. when it stresses logical realtionship between condition and result rather than telling us the actual result?
Conversely, should we use "if ... not" instead of "unless" where we want to make the outcome clear?