A spiritual master is someone in the line of disciples from God, and who has absorbed the knowledge that has been passed down. The Gauḍīya Vaiṣṇava tradition I try to follow is invigorated by the appearance of Śrī Caitanya Mahāprabhu from 1486 to 1534, as He was predicted to in many verses of the Vedas with abundant details. Many of His direct students are spiritual masters, and many people have subsequently dedicated their lives to the profuse teachings coming from Lord Caitanya. This is called the paramparā, disciplic succession.
There is no boundary of service, but devotional service is specifically motivated by love.
On the subject of the oneness of God's names, form, and personality, the best book I know of is Harināma Cintāmaṇi, which is available for free direct download here:
http://ebooks.iskcondesiretree.com/pdf/Gaudiya_Books%20/Bhaktivinoda_Thakura/Bhaktivinoda_Thakura_Hari_Nama_Cintamani.pdf
The Vedas prescribe how to properly learn them, and the most authentic translations I have found are in the books by Śrīla Prabhupāda. The above linked text is written by his guru's guru's guru. Most of what is popularly presented as the teachings of the Vedas is what I consider misinterpretations. The best modern understanding of the Vedas I have found is at www.ashishdalela.com, where I am regularly amazed and inspired.
There are more great books about these subjects than are possible to read in a lifetime.
I somehow previously overlooked your doubt about the legitimacy of referring to the God as a "He." I haven't used this app nearly enough to be proficient. The concept of masculine and feminine in the Vedas relates to enjoyer and enjoyed, and Kṛṣṇa is described as the ultimate enjoyer. The internal nature of Kṛṣṇa is feminine though, as His internal energy is the form of Rādhā, the ultimate feminine person. Kṛṣṇa is identified as God, but He worships Rādhā. Their combined form is Lord Caitanya, mentioned here in the first paragraph. It's also said God is male because the combined form of Rādhā-Kṛṣṇa is a celibate male.
CC Ādi 4.55: "The loving affairs of Śrī Rādhā and Kṛṣṇa are transcendental manifestations of the Lord's internal pleasure-giving potency. Although Rādhā and Kṛṣṇa are one in Their identity, They separated Themselves eternally. Now these two transcendental identities have again united, in the form of Śrī Kṛṣṇa Caitanya. I bow down to Him, who has manifested Himself with the sentiment and complexion of Śrīmatī Rādhārāṇī although He is Kṛṣṇa Himself."
Www.vedabase.com/en/cc/adi/4
Sorry for the delay, I was traveling for a time. I read through Ādi 4 and some descriptions of Śrī Caitanya Mahāprabhu. If I may say, it reminds me very much of the founder of the Baha'i religion, Bahá'u'lláh.
In many instances, particular philosophies elaborate on the existence of an individual who is thereby prophesied. In every instance, there is a particular individual which the followers of the particular philosophy attribute these traits to.
However, there is no unification between viewpoints present in the world. This fact alone indicates that prophecies have not been fulfilled, as the world is not unified in understanding. I do not aim to disrespect your perspective, but I would argue that Vishvambhar Mishra, who has come to be considered to be the prophesied Lord Caitanya, did not manifest in fulfillment of the particulars written of in a full and complete manner.
This brings me back to my original point: there is no such thing as a true master, as the true master is God. While it is possible to obtain awareness of many things, to such a degree where an individuals appears greatly aware relative to others, it is not possible to obtain complete awareness because there are infinite topics to become infinitely aware of. As you mention, there are even more books on the above subjects than are possible to read in a lifetime. Therefore, it is impossible for an individual to truly be a master. Following an individual opposed to following God, which is Everything and not limited to any one "thing" (for example, a person), is the only way to continue to grow. Otherwise, we are limited by the ignorance of the "spiritual master", who only knows what they know and nothing more. No matter what one knows, next to Everything it is nothing.
If there is such a thing as a spiritual master, they might as well be individuals who simply point at God. Not one part of God, but All. Meaning, pointing at God in Everything. Deferring to the true master rather than acting as one.