Sometimes you have to answer the question based on what you think the test writers were going for, even if the question isn't written perfectly.
I think you COULD try to make the crazy argument you made.
But, mathematically
"x is an integer between 10 and 15" means
x = 11, 12, 13, 14
So the idea of "between" is like bookends, but the bookends themselves are not eligible.
Grammatically, a ", wh-" modifier is a noun modifier that modifies the idea before the comma.
So saying "1765, when regulations first introduced" is modifying 1765.
You're trying to exploit the range of possibility in saying that A YEAR is when something first happened.
well, at what point during the year?On its own, I think you can attack that concept. But the way the entire sentence is structured, we're learning that trade doubled in the period between time X and time Y.
If time Y is when something else happened, we won't care. It will be outside the boundary line of the time period we're considering.
Once you (if you) accept that (E) is ineligible because it's outside the range of time we're considering, then you would certainly have to accept that (A) and (C)
could better contribute to an explanation.
The idea you expressed about (C), that efficiency "wouldn't necessarily" lead to more trade, is the wrong mindset for strengthen / weaken / explain.
The correct answer ideas for these tasks are nowhere near bulletproof. They just make something seem more plausible, or introduce doubt, or MAY be part of the explanation.
Hope this helps.