[susan53]

Sorry Brian - don't think I really understand the question. As I said above, the verb should not be in the simple past. The meaning would make no sense. There are two possibilities, with different meanings :

By the time the workers finished (A)... night had fallen (B) - ie night fell first, the workers finished second. B then A.
By the time the workers finished (a)... night was falling (B) - ie it had started but not yet finished. It was an on-going event.
B /........(A)............/

The simple past indicates a finished event. But if there are two, then they must logically occur either in 1-2 order or simultaneously. So you need the verbs that express those meanings.

If you want to say thet night fell very quickly and immediately after the workers finished , then you would need to change the adverbial :
As soon as the workers finished... (A) , night fell (B) - ie A then B.

As always, the choice of form has nothing to do with being "correct" or "idiomatic" or whatever. grammatical forms express different meanings. You need to choose the form which expresses the meaning which reflects the situation.