I understand by the notion of a purgatorial fire the purification of souls not by a created activity, but by their own unpreparedness to stand before our God, who "is a consumming fire" (Hebrews 12.29). The same glorious light of God that ravishes the blessed, purifies the unclean and destroys the wicked. The fire of purgatory is temporary because eventually the unclean are cleansed, and so come to full enjoyment of their salvation, not because purgatory is some locale where God roles up the welcome mat at the end of time.
St. Thomas Aquinas following Pope St. Gregory the Dialogist ("Dialogues", IV, 29) notes: "The souls of the damned ... always see the fire [of hell] as prepared for their punishment ... [, and] this vision is the immediate cause of their distress" (Summa Theologica, Supp. Q. 70, Art. 3, ad 8) St. Thomas also notes by quoting St. Augustine ("The City of God", i, 8) that: "'Even as in the same fire gold glistens and straw smokes, so in the same fire the sinner burns and the elect is cleansed.' Therefore the fire of Purgatory is the same as the fire of hell: and hence they are in the same place." (Summa Theologica Supp., Appendix II, Art. 2)
The Bible is perfectly clear that the damned will "be tormented with fire and brimstone in the sight of the holy angels and in the sight of the Lamb." They are with Jesus too, not off in some seperate place. They simply do not enjoy being with Him, since they hate Him and their fellow man. Therefore, those undergoing purification will also be in the presence of Jesus, and will receive the same punishment, though in a different spiritual quality, since they are among the saved, and not the damned.
St. Mark of Ephesus, of all people, neatly sums this up in his sermon against the purgatorial fires: "But some must be cleansed in the very departure from the body (as St. Gregory the Dialogist literally shows); while others must be cleansed after the departure from the body, before they come to worship God and are honored with the lot of the blessed, or — if their sins were more serious and bind them for a longer duration — they are kept in hell, but not in order to remain forever in fire and torment, but as it were in prison and confinement under guard. All such ones, we affirm, are helped by the prayers and Liturgies performed for them, with the cooperation of the Divine Goodness and Love for mankind. And so, we entreat God and believe to deliver the departed (from eternal torment), and not from any other torment or fire apart from those torments and that fire which have been proclaimed to be forever."
It is difficult to see the difference between what St. Mark says, and what St. Thomas says, seeing they both say the place of cleansing fire after death is hell. St. Thomas merely specifies that the state of being purified temporarily in hell is called purgatory, while St. Mark objects to this name over its then current connotation (as in Dante) of a third place besides Heaven and Hell, the apparent conception (by some) of it containing a created fire of purgation seperate from the divine energies.
And this comports perfectly to the Roman Liturgy. In the Collect of the Funeral Mass, we pray: "... do not then let him undergo the pains of Hell ..." And we sing in the Offertory: "... free the souls of all the faithful departed from the throes of Hell, from the fathomless pit. Snatch them from the lion's mouth, lest Hell engulf them, and they sink down into darkness." And the constant refrain of the versicles in the Office of Matins and Lauds of the Dead and in the Mass: "V. From the gate of hell. R. Deliver his soul, O Lord." Purgatory is never mentioned by name once, not because it is not, but because it is not different from hell, which is mentioned quite a bit.
Those who don't repent also don't believe, so who exactly is "flamed" to pristine purity in this mysterious state called the Purgatory? For, if they feel God's fire not as loving warmth but as unbearable burning, it is because they hate Him.
The way you describe the Purgatory leaves two possibilities -- either God purifes those in the Purgatory through His will and against theirs, or they change their mind once Love sears their wicked souls. So regardless which one you choose, it seems not to be a conversion in the heart but extortion under duress.
I appreciate your efforts to defend us Latins, but I'm afraid you're misinterpreting St. Thomas. He, like most of the scholastics, taught that the fire of Purgatory/Hell was a material, corporeal fire. The statement that the fire is "eternal in substance" does not prove what you want in any case - he merely means that it will last forever. "Aeternus" can simply mean "enduring, permanent, endless" - the point is that the fire of hell is "everlasting", even though the souls in purgatory will experience it only for a temporal duration. In the Summa Contra Gentiles, lib. 4 cap. 90 he discusses how the corporeal fire of hell can cause incorporeal substances (that is, souls) to suffer. See also Quaestio disputata de anima, art. 21, "Vicesimoprimo quaeritur utrum anima separata possit pati poenam ab igne corporeo".
You're also misunderstanding Mark of Ephesus. When he says "they are kept in hell, but not in order to remain forever in fire and torment, but as it were in prison and confinement under guard", he doesn't mean that they are tormented by fire but just not "forever". He says elsewhere "We affirm that neither the righteous have as yet received the fullness of their lot and that blessed condition for which they have prepared themselves here through works, nor have sinners, after death, been led away into the eternal punishment in which they shall be tormented eternally. Rather, both the one and the other must necessarily take place after the Judgment of that last day and the resurrection of all. ... nor have the second part yet been given over to eternal torments nor to burning in the unquenchable fire."
Fr. Seraphim Rose describes Mark's doctrine, from the homilies against purgatorial fire given at Florence:
In the Orthodox doctrine, on the other hand, which St. Mark teaches, the faithful who have died with small sins unconfessed, or who have not brought forth fruits of repentance for sins they have confessed, are cleansed of these sins either in the trial of death itself with its fear, or after death, when they are confined (but not permanently) in hell, by the prayers and Liturgies of the Church and good deeds performed for them by the faithful. Even sinners destined for eternal torment can be given a certain relief from their torment in hell by these means also. There is no fire tormenting sinners now, however, either in hell (for the eternal fire will begin to torment them only after the Last Judgment), or much less in any third place like "purgatory"; all visions of fire which are seen by men are as it were images or prophecies of what will be in the future age. All forgiveness of sins after death comes solely from the goodness of God, which extends even to those in hell, with the cooperation of the prayers of men, and no "payment" or "satisfaction" is due for sins which have been forgiven.
Because of this rejection of the purgatorial fire on the part of the Greeks, the Council of Florence (wisely) purposefully refrained from defining anything on the question. It now appears that the Orthodox here want to impose their own particular opinions on us, though. Can't we just leave the matter free, as it was in the time of the Fathers?
If a man departs this life with lighter faults, he is condemned to fire which burns away the lighter materials, and prepares the soul for the kingdom of God, where nothing defiled may enter. For if on the foundation of Christ you have built not only gold and silver and precious stones (I Cor., 3); but also wood and hay and stubble, what do you expect when the soul shall be separated from the body? Would you enter into heaven with your wood and hay and stubble and thus defile the kingdom of God; or on account of these hindrances would you remain without and receive no reward for your gold and silver and precious stones? Neither is this just. It remains then that you be committed to the fire which will burn the light materials; for our God to those who can comprehend heavenly things is called a cleansing fire. But this fire consumes not the creature, but what the creature has himself built, wood, and hay and stubble. It is manifest that the fire destroys the wood of our transgressions and then returns to us the reward of our great works. (Origen, P.G. , XIII, col. 445, 448)
It is one thing, tortured by long suffering for sins, to be cleansed and long purged by fire; another to have purged all sins by suffering. It is one thing, in fine, to be in suspense till the sentence of God at the day of judgment; another to be at once crowned by the Lord. (St. Cyprian, Letter 51:20)
For it is said: He himself however will be saved, though as if by fire [1 Cor 3, 12 15]. And since it is said, he will be saved, that fire is disdained. So for anyone to be evenly saved by fire, that fire however will be graver than anything a man can suffer in this life. (St. Augustine, in Ps. 37, n. 3)
This punishment under the earth will await those, who, having lost instead of preserving their Baptism, will perish for ever; whereas those who have done deeds calling for temporal punishments, shall pass over the fiery river and that fearful water the drops of which are fire. (Eusebius Emissenus)
When he has quitted his body and the difference between virtue and vice is known he cannot approach God till the purging fire shall have cleansed the stains with which his soul was infested. That same fire in others will cancel the corruption of matter, and the propensity to evil. (St. Gregory of Nyssa, P. G., XLVI, col. 524, 525)