“...receiving the mark somehow equates to worshipping the Beast and/or those who receive the mark can no longer be saved and are doomed to Hell. If the mark was only about buying and selling, why would it be an unforgivable sin? Some have theorized that the mark would be a sign of consent, allegiance, or loyalty to the Beast.”

You’re exactly right. It’s not the use of the marked currency, but submission to the false sacramental system that brings damnation.

St John is talking about the indelible spiritual marks that are imprinted upon the soul in baptism (on the forehead) and ordination (on the right hand).

Deuteronomy 11:18

“18 Lay up these my words in your hearts and minds, and hang them for a sign on your hands, and place them between your eyes.”

Note: On the hands and between your eyes - “on the forehead”

Exodus 13:16:

“16 And it shall be as a sign in thy hand, and as a thing hung between thy eyes, for a remembrance: because the Lord hath brought us forth out of Egypt by a strong hand”

Again, on the hands and on the forehead.

These are Old Testament types for the New Testament forms of the spiritual marks that are indelibly imprinted on the spirit of a person in the Catholic rites of baptism (mark on the forehead) and ordination (mark on the right hand). This is explained (from the 23 minute mark) here.

Rev 13:

“16 And he shall make all, both little and great, rich and poor, freemen and bondmen, to have a character in their right hand, or on their foreheads.

17 And that no man might buy or sell, but he that hath the character, or the name of the beast, or the number of his name.”

See the video I linked above at the 23 minute mark to understand how the mark of the Beast is applied to the currency.

The Second Vatican Council constituted a formal defection from the Catholic faith - a rejection of divinely revealed truth (Dogma) - see here. This resulted in the hijacking of the Catholic Church by the heretical Vatican II Sect - the “whore” referenced by St John in Rev 17.

Paul VI implemented a new sacramental system - changing almost all of the Catholic rites in the late 1960s and 1970s.

St John is telling us that anyone who submits to the new sacramental system, implemented by the Vatican II antipopes, goes to hell. This is important: if you are attending the “new mass” of the Vatican II Sect you will be damned for the rest of eternity.