From my JLPT N1 grammar text, explaining special uses of the ている form, in describing 事実に反すること; the text explains that ている marks the sentence more emphatically as non-reality, than the use of 過去形. However, it's not clear to me whether the final verb needs to match the ている form or not. It does agree in both their examples, one of which is:
こんなに大変な仕事だとわかっていたら、断っていただろう。
Would
こんなに大変な仕事だとわかっていたら、断っただろう。
be grammatical? If so a different nuance? Or is it ungrammatical?
First of all, こんなに大変な仕事だとわかっていたら、断っただろう。 is a natural sentence. And, 断っていた/断った do not make much difference, in this particular case at least.
That said, more microscopic analysis would show us some difference in nuance. 断った is focusing on the action of 断る, while 断っていた targets more on to the consequence of the action.
If the core interest of the speaker is more on the (non-real) action, he/she would say 断っただろう, otherwise they use 断っていただろう.
That's like, if we have drawn a picture for each situation, 断っただろう would be a picture where someone is closing the door with a slap noise , while 断っていただろう would be a picture of the door already closed.
As I'm not linguist I'm not sure, but possibly that's like what is called 完結相 or perfective aspect in linguistics.
Edit: The analytics above is based merely on my perception as a speaker, and is not on that authentic theory.
