Question about modifiers

without the citation of source, many tutors will pass it.

Let me address your queries as precisely as possible-

Can the location of second modifier 'a tiny beach town that sits approximately 75 miles.....' refer to the main subject?

A second modifier in that particular location can modify subject.

However, it will be required to meet some criteria: there needs to be some good algorithm in place that captures the essence of GMAC's stance on this issue and that, thus, can be backed by instances in official material.

The second initial modifier will have to be parallel with first initial modifier.
If it is parallel to the first initial modifier, then ideally there needs to be a parallel marker.
There are, however, instances in official material where a parallel marker, typically and, is dropped conveniently.
and is dropped because it hardly effects the easy readibility.
Droping and, therefore, can be cited as an issue of style and not the issue of meaning or grammar.

However, two answer choices will never differ only on style issue.

In this particular case, the second initial modifier cannot refer to the subject because
1)the contextual information makes it crystal clear that it refers to the noun in first initial modifier
2)first initial modifier is past participle phrase whereas the second modifier is appositive; We don't have this precedence ( participle phrase made parallel to appositive with and dropped, all happening in the initial part of the sentence) in official material( to the best of my knowledge ).

Is there any rule about 2 successive modifiers?

two successive modifiers with and dropped is permissible in GMAT SC.
However, it is observed that first modifier is typically short in length as compared with the length of second modifier( as would have been the case here)- again it is a style issue( easy readibility and has nothing to do with grammar or meaning)

P.S:
Please understand that initial modifiers have purpose: the information they contain needs to be related with the information in the main clause. The random positioning is not allowed.

In this particular case, the initial modifier Located in Carpinteria, California has a close relationship with the information in main clause. If there is no relationship, then GMAC typically inserts and to show that these two pieces of info. have no relationship, and thus makes both pieces of info two seperate modifiers or two seperate sentence.


P.P.S:
E. a tiny beach town that sits approximately 75 miles north of the city of Los Angeles and overlooks the mountains, the house is just a few short blocks from the ocean
a tiny beach town that sits kind of communicates that there are more than 1 tiny beach towns in Carpinteria, California, and we are talking about the one that sits 75 miles north of LA.
As it requires the knowledge outside the domain of what has been communicated in this sentence, it is a non issue in a situation you will find yourself when writing the test.

In the OA, there are 2 modifiers, Can the second modifier refer to the main subject?

An APPOSITIVE is a noun or noun phrase that serves to explain or define another noun or noun phrase.
An appositive preceded by a comma serves to explain the noun or noun phrase directly before the comma.
E: Located in Carpinteria, California, a tiny beach town that sits approximately 75 miles north of the city of Los Angeles and overlooks the mountains, the house is just a few short blocks from the ocean.
Here, the appositive in blue is preceded by a comma and thus serves to explain the noun phrase directly before the comma (Carpinteria, California).