I know that relative clauses are generally made with the following format.
sentence/verb/adjective + noun
Strictly speaking, a relative clause refers only to the part before the modified noun. 本をよんでいる学生 is not a relative clause although it has a relative clause. The grammatically correct way to explain this phrase is "本を読んでいる is a relative clause which modifies 学生".
(On this site, you may see people say "本を読んでいる学生 is a relative clause" for the sake of brevity, but technically speaking that's not the most correct explanation.)
Do you understand what a clause (節) is in general? Very roughly speaking, it's a predicate associated with corresponding subject/object/etc (see this). The main clause of a sentence refers to the outermost clause (=subject-predicate pair). In your example sentence:
- Main clause:
The student is Michiko
- Relative clause (modifying 学生/student):
who is reading a book over there
referring to the noun part of a relative clause as a 'main clause'.
"The noun part of a relative clause" is not part of the relative clause in the first place. In your example, 学生 belongs to the main clause.
where the noun/'main clause' ends (therefore the end of the total relative clause construct) in Japanese sentences.
In Japanese, when a relative clause ends, the main clause (re)starts (unless several relative clauses are deeply nested). In a regular Japanese sentence, the main clause ends always at the end of the sentence.
In English, a relative clause starts with a relative pronoun (that/which/who/where/etc, although that may be omitted), but the modified noun before it is not part of the relative clause.
Is the main clause in the relative clause construct a actual clause?
This question doesn't make sense to me. The main clause refers to the outermost clause in a sentence. A relative clause never contains a main clause. (A sentence may have two or more main clauses. See: compound sentence)
is the entirety of 学生はみちこさんです the main clause which the relative clause modifies, or is it only the noun that is being modified
A clause is a noun? If you know what a clause is, you don't have to ask a question like this. A clause is not a noun, but basically a subject-predicate pair. And a relative clause modifies a noun, not a clause!
- Relative clause: 返事をした (modifies 女の子)
- Main clause 1: 女の子がこちらに近づき
- Main clause 2: (女の子が)僕の手にナイフを握らせる
- Relative clause: 荒野を渡る (modifies 風)
- Main clause: 風よりもなおアインの声は冷たかった
Note that each clause (main or relative) has one and only one predicate (an adjective or a verb).