Learn English – When you have a sentence containing a list separated by commas, do you need to repeat the verb before a preposition

"Popular among" and "popular with" are both correct. In this case, I would suggest "popular with".

"Popular with" is usually used for people, e.g. "Popular with the girls", "Popular with the working class..."

"Popular among" is usually used with ideas among people, eg. "The new policy is not going to popular among the members of the party...."

I, The McGraw-Hill Handbook of English Grammar and Usage, and Grammatically Correct, 2nd Edition agree with you. "Before" in the first example sentence is a subordinating conjunction, not a preposition. It is a subordinating conjunction because it begins an adverb clause, "before the concert starts". The McGraw-Hill Handbook provides the following example:

I had finished my popcorn before the movie even started.

The Handbook indicates that "before" is a subordinating conjunction here. (And you'll notice that the sentence is coincidentally similar to the one you've presented.) Grammatically Correct also explicitly lists "before" in its list of subordinating conjunctions.

"Although he didn't know the answer" is also an adverb clause, so "although" is also acting as a subordinating conjunction.

"After the concert" is a prepositional phrase, unless the writer intended an understood (sorry!) "ends" at the end of the sentence, in which case the formation would be an adverb clause. As the sentence is written, however, the formation is a phrase.

"Out of the box" is definitely a prepositional phrase, as there is no verb present at all.

Lastly, I would also call "before" in the last example an adverb modifying "seen", unless the writer intended an understood "now" at the end of the sentence. Again, though, going by what is written, "before" is an adverb.