Are you sure that Paradise and heaven are the same thing?
You write of "paying for" the stain of sin. I'm not sure we are taking about the same thing. Jesus has paid in full the debt of our sin. We deserve death. He took the death we deserve. Nevertheless there are certain disabilities consequent upon vice, just as there are abilities which follow the practice of virtue -- no matter how imperfect our motivation for those practices might be.
While miracles happen, the mere act of handing yourself over to Jesus and accepting Him as your Lord and savior does not automatically and inevitably make you an unselfish person, for example. If you tended, say, to the hysterical, to wanting others to notice and discuss amongst themselves and with you what a fabulous, warm, and fuzzy person you are, it's not a sure thing that that habit will die with our conversion or, we maintain, with your death.
Purgatory, as I see it, is where you work off those habits and develop good habits. You are admitted to Purgatory, the front porch of heaven (for yet another metaphor) through the all sufficient merits of Christ. And, as I said, everyone there rejoices because they are saved already because Jesus has already, as you say, paid the full price of their sins.
Saying that something is proved in Scripture or not is always tricky. I find that if all I have is Scripture I do not necessarily come up with Purgatory as a notion. But WITH the doctrine of Purgatory I find many questions answered and passages of Scripture take on what is to me a fuller meaning.
But to be redundantly clear, there is a difference between removing the stain and paying the price. Consequently we can agree with you that forgiveness is supplied through Christ's work alone. We are not saying much about paying a debt except in a sort of double metaphor.
What we are addressing in the doctrine is the fact that people sin a after they are saved. So while something is washed away, there is still something which seems to need a remedy.
What do you think of the sins someone commits after they are saved? My experience is that my relationship with Jesus and my confidence in His saving love drives me to greater awareness of my sinfulness -- and that enables me to begin to pluck out, always with His help (since the root of the problem is usually not asking for His help) the bad habits I have given myself by frequent indulgence in bad deeds.
V:10 and in Him you have been made complete. Are we not made complete in Him because we still sin, then His death was not good enough to do as He claimed.
V:8 warns us of what happens when we put our faith in "tradition of men" instead of Him alone. This what the RCC tradition is.
Col 2:6 Therefore as you have received Christ Jesus the Lord, so walk in Him, Col 2:7 having been firmly rooted and now being built up in Him and established in your faith, just as you were instructed, and overflowing with gratitude. Col 2:8 See to it that no one takes you captive through philosophy and empty deception, according to the tradition of men, according to the elementary principles of the world, rather than according to Christ. Col 2:9 For in Him all the fullness of Deity dwells in bodily form, Col 2:10 and in Him you have been made complete, and He is the head over all rule and authority; Col 2:11 and in Him you were also circumcised with a circumcision made without hands, in the removal of the body of the flesh by the circumcision of Christ; Col 2:12 having been buried with Him in baptism, in which you were also raised up with Him through faith in the working of God, who raised Him from the dead. Col 2:13 When you were dead in your transgressions and the uncircumcision of your flesh, He made you alive together with Him, having forgiven us all our transgressions, Col 2:14 having canceled out the certificate of debt consisting of decrees against us, which was hostile to us; and He has taken it out of the way, having nailed it to the cross.