I had always supposed for 〜てもらう sentence the subject has to be the third person or the second person. Is that what you were saying?
Again, "subject" is ambiguous. "
The subject of 〜てもらう sentence" is the recipient of the favor, but you must refer to
the subject of the action (試す or 分かる), i.e.,
the agent (動作主) by "subject".
As for your question, I wrote;
the subject of 試す and 分かる can be third person(someone else other than the speaker "I/we" and the addressee "you") in お試し頂けたら分かってもらえるでしょう
, thus, what I said is another interpretation that the agent/the subject of 試す and 分かる is not second person "you" can be possible
in that sentence.
I had supposed "お + masu stem of verbs + になる" is an honorific form.
Yes, "respectful" or "honorific" is more appropriate.
Does "お + masu stem of verbs + いただく" apply to all verbs for converting to humble form?
Yes, as same as "お + masu stem of verbs + になる".
What about お or ご + the Polite Form + する (Instead of (し)ます)? I have come across it on
The Honorific Form, the Humble Form, and the Polite Form
I don't think "polite form" is an appropriate term, since that form (e.g. 待ち, 教え, 泳ぎ or stem of -suru verbs) is not "polite form" . It should be "-masu stem", i-form, 連用形 etc.
The direction of respect is different. The respect is towards the agent in "お + masu stem of verbs + いただく" (or "ご + stem of -suru verbs + いただく"), while it's from the agent towards second or third person in "お + masu stem of verbs + する" (or "ご + stem of -suru verbs + する"). Thus, the agent must be the speaker or their in-group member(s) in "お + masu stem of verbs + する".
明日までお待ちいたしております。(いたす is a humble verb of する)
The agent(the one who waits) is the speaker, and respect is from the agent(= the speaker) to the addressee "you".
明日までお待ちいただきます。
The agent(the one who waits) is the addressee "you", and respect is from the speaker to the agent (= the addressee "you").
兄が街をご案内します。
The agent(the one who guides) is the speaker's brother, and respect is from the speaker to the addressee "you".
昨日先方に(or から)ご回答いただきました。
The agent(the one who answered) is the other party (customer/client), and respect is from the speaker to the agent.
Notice that respect is from the speaker towards someone else in all expressions, after all.