“If, as according to the RCC, you commit a mortal sin and it’s unconfessed to the priest, you do not go to Heaven. The real meaning: one would go to Hell.”
No. It is not that the sin must be confessed to a priest (although that would be best if circumstances allow it) - for that might be impossible because of circumstances. The sin must be renounced and repented of. God sent priests for confession (John 20:19-23), but God knows that not everyone has access to a priest at all times.
A man who had committed a mortal sin, and genuinely had contrition for it can, if he were in a situation where confession to a priest were impossible, commit himself to the mercy of God in perfect contrition. I have no reason to doubt God’s mercy in that situation. I also have no doubt that God has been merciful enough to give us confession to help us avoid that situation in the first place.
What we see here again is that you’re attacking something you apparently don’t even understand because, if you did understand it, then you would not have made the mistake you just made.
1484 “Individual, integral confession and absolution remain the only ordinary way for the faithful to reconcile themselves with God and the Church, unless physical or moral impossibility excuses from this kind of confession.” There are profound reasons for this. Christ is at work in each of the sacraments. He personally addresses every sinner: “My son, your sins are forgiven.” He is the physician tending each one of the sick who need him to cure them. He raises them up and reintegrates them into fraternal communion. Personal confession is thus the form most expressive of reconciliation with God and with the Church.
1452 “For contrition to forgive mortal sins it must arise out of our love of God. “When it arises from a love by which God is loved above all else, contrition is called ‘perfect’ (contrition of charity). Such contrition remits venial sins; it also obtains forgiveness of mortal sins if it includes the firm resolution to have recourse to sacramental confession as soon as possible””.
Welcome to the Biblical understanding of asking Christ for the forgiveness of our sins. No earthly priest needed.