To find the main verb in an English sentence, first find all the Verb Phrases. Each one defines a clause, because only clauses have Verb Phrases.
The main Verb Phrase of the whole Sentence is the Verb Phrase that all the other clauses modify; this makes the clause containing it the Main Clause. So the main verb in the Main Clause in a sentence is the main verb of that sentence.
In each Verb Phrase, the main verb is the last verb; all the other verbs in that Verb Phrase can be ignored. The rule is then that the last verb in the Verb Phrase of the main Clause is the main verb of the sentence.
In the sentences presented,
- Member is allowed to change himself back
- I want to go to school
- He needs to stop
In (1), is allowed and to change himself back are the verb phrases, and since the second one is the object of the first, that makes the first one the main verb phrase, and the main verb in it is is. (The real predicate in this sentence is actually allowed, but that's a predicate adjective, not a verb.)
In (2) want and to go to school are the VPs, and the second modifies the first again, so the main verb is want.
In (3) needs and to stop, same story. Main verb, needs.