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.
They are both grammatically correct.
With the same consideration in both cases is an adverbial prepositional phrase modifying the verb treat.
That I would a friend is an adjectival clause modifying consideration. The verb in the clause is implied, and in full form would be that I would treat a friend. While the preposition could be added along with the verb, it is not essential. If it were added, the possibilities are
Am I treating this stranger with the same consideration that I would treat a friend with?
Am I treating this stranger with the same consideration with which I would treat a friend?
While both are grammatically correct (assuming you have surrendered on the avoidance of terminal prepositions), they sound clumsy.
The originals sound fine.
Best Answer
I think that, technically, you should say
To be honest though, people don't speak like that anymore. I would, however, write like that, particularly in a relatively formal setting, such as an essay, paper or publication.