Scripture alone doesn’t support itself? Nonsensical.
Depends on what you say the doctrine of sola scriptura is. If it is that scripture by itself is entirely sufficient for all doctrine, etc... no, scripture doesn't say that.
I suspect the Scriptural basis relates to passages such as
1Co 3:10-21
(10) According to the grace of God which is given unto me, as a wise masterbuilder, I have laid the foundation, and another buildeth thereon. But let every man take heed how he buildeth thereupon.
(11) For other foundation can no man lay than that is laid, which is Jesus Christ.
(12) Now if any man build upon this foundation gold, silver, precious stones, wood, hay, stubble;
(13) Every man’s work shall be made manifest: for the day shall declare it, because it shall be revealed by fire; and the fire shall try every man’s work of what sort it is.
(14) If any man’s work abide which he hath built thereupon, he shall receive a reward.
(15) If any man’s work shall be burned, he shall suffer loss: but he himself shall be saved; yet so as by fire.
(16) Know ye not that ye are the temple of God, and that the Spirit of God dwelleth in you?
(17) If any man defile the temple of God, him shall God destroy; for the temple of God is holy, which temple ye are.
(18) Let no man deceive himself. If any man among you seemeth to be wise in this world, let him become a fool, that he may be wise.
(19) For the wisdom of this world is foolishness with God. For it is written, He taketh the wise in their own craftiness.
(20) And again, The Lord knoweth the thoughts of the wise, that they are vain.
(21) Therefore let no man glory in men. For all things are yours;
We know He will bring Israel back to Him, as tested by the fire, which IMHO refers to the Great Tribulation after the Church Age. I also do not think the Church is that much more savvy and devout than historical Israel and we are likely to lapse just as much, if not more-so than Israel against His Will.
In the eternal Kingdom, and in the resurrection body we no longer have our old sin natures, they have been removed or sanctified at some time. Isn’t Purgatory simply a place where the old sin nature is berned up and removed prior to our entrance to the abode of God for eternity future?