I agree to this point. A meaning of "manifestation is "making plain" or "making visible".
No, vv8-10 are a list of manifestations. Nine different ways the gift of holy Spirit is "made plain", or "made visible".
Earlier I mentioned the analogy of the Swiss Army knife. Another example might be a light bulb. A light bulb has two manifestations: light and heat. They are not "gifts" that the light bulb gives, they are different ways the light bulb can manifest itself, or "make itself plain".
I do not know of any other verse in the Bible that defines or describes any other manifestation besides those listed in 1 Cor 12:8-10. Do you?
They are not gifts, they are manifestations OF the gift of holy spirit.
And the bible distinctly states that the manifestation of the spirit is given to EVERY man (meaning Christian) (1 Cor 12:7).
I mean that every Christian has the gift of holy Spirit, and that every Christian has the ability to manifest that gift in nine different ways. The manifestation of "gifts of healing" is the only manifestation that is called a "gift" because healings are ALWAYS a gift.
When the apostles spoke in tongues on the day of Pentecost, who did the speaking? The apostles did. When Peter heard Cornelius and his family speaking in tongues in Acts 10, who did the speaking? Cornelius and his family did. When people prophesy, who does the speaking? The prophets do. "The spirits of the prophets are subject to the prophets" (1 Cor 14:32).
Christians are responsible for the operation of the manifestations of the gift of Holy spirit. Because of the power of the Holy Spirit dwelling in you, you have the ability to speak in tongues whenever you want to. If you don't know you can, you won't.
That being said, several of the manifestations require revelation from God. You can't just demand a miracle to be performed. You can't demand that someone be healed. God has to give you the go-ahead, and then it's up to you to carry it out. An example of that is in Acts 14:
8) And there sat a certain man at Lystra, impotent in his feet, being a cripple from his mother's womb, who never had walked:
9) The same heard Paul speak: who stedfastly beholding him, and perceiving that he had faith to be healed,
10) Said with a loud voice, Stand upright on thy feet. And he leaped and walked.
Paul "perceiving that he had the faith to be healed" was revelation from God (word of knowledge and word of wisdom, two more of the manifestations).
Again, we have an Adversary, and I believe he worked (and still works!) overtime to minimize and corrupt the truth of what God made available to Christians through the gift of holy Spirit.