If I wanted to make this totally clear and unambiguous I would use
... will be distributed pro rata based on number of shares held.
In this sentence, pro rata is an adverb.
The preposition to that you have suggested links to the verb distribute. With this verb, to links to the target of the distribution and typically the preposition by introduces the manner of the distribution.
distribute food by truck to a Somali refugee camp
You could say
will be distributed pro rata by shares held.
however, I want it to be clear that by matches the verb rather than the adverb.
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.
Best Answer
In all of your examples it's the preceding phrase, not the word whether, that controls the following word(s):
I'm confused as to whether a preposition is required before whether.
I'm confused as to the nature of your question.
I'm confused how often I need to ask the question. (I'd prefer confused about, but about is commonly omitted in practice, especially informally.)
I haven't decided on whether to put a preposition before whether.
I haven't decided on a dress to wear.
I haven't decided to take the job.
I don't know about whether a preposition is required before whether.
I don't know about history.
I don't know biology.
(In your sentence, I'd always omit about.)
There's no news as to whether Jane will show up to today's meeting.
There's no news as to the outcome of the election.
There's no news concerning the baby.
We haven't received an update on whether the party will arrive on time.
We haven't received an update on your progress.
We haven't received an update to the software.
(We always receive updates on or to things. On means "about a topic." To means the thing has been changed.)