The insect, famed for its remarkable annual migrations, faced declines

Dear aditya8062
I'm happy to respond.

First of all, please do NOT use the quote boxes if you are simple quoting a sentence from another source, such as the OG. Simply use a different color or something of that sort. The quote boxes are to demonstrate what a previous GC user has said.

Also, just for reference --- this is a common mistake for non-native speakers --- spaces never come before periods & commas, as you have in your writing above. They always come after --- one space after a comma, and two after a period. Also, you need to capitalize the first word of each sentence. Mistakes of this sort on your business school application would be a huge red flag, perhaps unfairly indicating weakness in your verbal skills. You may know all this already, and perhaps you were just being casual on GMAT Club. This is your opportunity to practice writing absolutely flawless English: please do not underestimate how valuable this opportunity is. Everything you write here should be of the highest possible quality, so that you can get valuable feedback on your writing.

As to your question: yes, a sentence is certainly a run-on if two independent clauses are separated only by a comma. The problem with the sentence in the prompt is not that a a subject is missing. It would be perfectly correct parallel structure to omit the same subject for a second verb.
The insect faced decline, and now is the focus of ...
The candidate won his previous election in a landslide, but now is trailing in the polls.
Those sentences are 100% correct --- because of the parallel structure, we can drop the subject in the second branch. See:
https://magoosh.com/gmat/2013/dropping-c ... -the-gmat/
The parallel structure absolute depends on the conjunction. If we omit the conjunction from either, these sentences become wrong:
The insect faced decline, now is the focus of ....
The candidate won his previous election in a landslide, now is trailing in the polls.
Now, to some extent, it is a matter of semantics whether to call these incorrect sentences "run-on sentences." I would call them "run-on sentences," precisely because they are incorrect for exactly the same reason that it is incorrect to juxtapose two independent clauses. In either case, simply adding a conjunction would fix the problem.

An adverbial clause, like any clause, would need to have a clear subject and clear verb. An adverbial phrase, such as a participial phrase, would not have a full verb. The structure "now is the focus of ..." cannot possibly be a proper phrase or clause, because it has a full verb "is" and no subject. It can only be the second branch of a parallel construction in which the subject is stated explicitly in the first branch.

The GMAT sentence about Lake Baikal is entirely different. Notice that you appended an extra verb, which makes the sentence wrong. That final "does" was not part of the OA of that question ---- if we put a verb there, it would have to be "do." The clause following the comma is 100% correct:
more than all the North American Great Lakes combined
Unlike the example from above, this has a legitimate subject and a verb implied by the parallel structure. Furthermore, this begins with an appropriate transition to the second branch of parallelism, the words "more than."

Does all this make sense?
Mike