The sin of blasphemy against the Holy Spirit - is - salvation itself. The work of Salvation is from the Holy Spirit, in that “man is dead” (spiritually dead, with the body catching up in a few decades, later).
Being that everyone is unable on his or her own to accept Jesus as their personal Savior (because of mankind’s sin nature, resident in him from birth) - the Holy Spirit intervenes to allow the person’s choice to be made, and also the Holy Spirit speaks directly to each person’s spirit in giving testimony to that person that God “is”, that He had made everything that one sees (of creation) and that God has provided a way of salvation, if they seek it out.
For the person to BLAPSHEME the Holy Spirit, is to say that the Holy Spirit is a LIAR and what the Holy Spirit is testifying to that person’s spirit is NOT TRUE.
THAT will not be forgiven, because that is rejection of the Saving work of God, through His Son Jesus.
IN OTHER WORDS - that person has rejected Salvation, and the rejection of Salvation IS THE ONE THING THAT WILL NOT BE FORGIVEN ... and will result in being cast into the lake of fire and being tormented forever and ever.
You have a firm grasp of what the Holy Spirit is and does. I personally have felt the Holy Spirit move me and I have felt the hand of Angels keeping me from certain death. I know these things are true as well as I know the lines on my face. I was responding to your statement:
“I’m not saying that there is theoretically anyone who cannot be saved.”
I am saying that CHRIST HIMSELF stated that there is one sin... once committed... that will never be forgiven.
GOD bless you and yours and Happy New Year!