That is what "covers" means in this context, that your trespasses were atoned for.
but if He had only been forsaken by men, (as acknowledged in Psalms 22) then why is it that He cried out and addressed God and the Holy Spirit?
Because He alone is the Human being that is upon the cross, as a person who is righteous and holy. The forsaken-ness, isn't that God has caused Him to die spiritually, the forsaken-ness is that He is the one bearing the penalty. Read Psalms 22, which is a Messianic Psalm about this and the Psalm He is fulfilling. Look at verse 24.
For he hath not despised nor abhorred the affliction of the afflicted; neither hath he hid his face from him; but when he cried unto him, he heard. (Psalms 22:24, KJV)
YHWH didn't forsake Him in the sense that JDS teaches, but heard Him and did not despise or abhor His affliction.
Also, I didn't day He was only forsaken by men. I said that He did not die spiritually, as is being read into the text. You are assuming that forsaken is dying spiritually, which it is not. It is speaking to the loneliness and work of the Messiah, who alone is bearing the penalty for atonement.
if They too had not forsaken Him at that moment, would He who had always known and acted with Them have ever accused Them of doing so?
Not sure what you are asking. He already had told them they would scatter and leave the Shepherd, if that is what you are getting at.
never in the whole of history until that moment had the trinity not been together and acted together as one.
This is what I mean, to make this statement one has terribly misunderstood the nature of YHWH. There is one YHWH, not three. YHWH can not be divided and broken into three, that is impossible.
He changes not, as the scripture clearly tells us. The only thing that is not eternal is the humanity that He took in the incarnation, and that was what was needed to make atonement for His people. The Eternal Spirit can not be separated from YHWH.
Hear, O Israel:
The LORD our God [is] one LORD: (Deuteronomy 6:4, KJV)
How much more shall the blood of Christ, who through the eternal Spirit offered himself without spot to God, purge your conscience from dead works to serve the living God? (Hebrews 9:14, KJV)