Not long before ... , inversion?

Question

I got this question in my English exam this morning and have been wondering about it:

So my questions are:

1. Can "Not long before [something happened/an event/a point in time]" be seen grammatically as "Before [something happed...]?

If the answer is yes, than the sentence will become "Before the interval, ____________.". Therefore, I can totally choose B. so that the complete sentence will be "Before the interval, the critic came to the theater.", right?

2. If the answer is D., the sentence will become "Not long before the interval, did the critic come to the theatre."

However, I think the comma is absolutely abundant. So my questions are:

2.1. Can we use a comma before a 'subject-auxiliary verb inversion'?

2.2. Is "Not (so) long before/after ..." counted as a way of beginning an inversion?

Thank you. And if there is any grammar mistakes above, please tell me too.

Answer 1

The reason for comma after

Not long before the interval,...

is that you can put comma after an introductory element (Purdue University).

"Not long before" is still "before" and makes no "inversion," so the sentence is still in the past tense and the correct answer is B) "the critic came to the theater."