The CCC passage here is not defining a dogma, which is outside the nature of a catechism to do anyway. It is merely reiterating previous dogmas from the councils it references. In order to understand what is meant by “entirely different,” you must see how those councils explain the two states differently. The CCC does not expound or explain what it means by “entirely different.” I explained above how this idea can be understood in accordance with tradition as that is how the Church builds upon dogmas. Unless there is evidence otherwise from the councils of Florence or Trent that define purgatory in such a way as you are interpreting it, then you likely need to rethink your interpretation.
The same thing happens when we read Scripture. Often times we find a passage that fits with the way we perceive something and then interpret the passage to mean that when the passage itself may have never intended such. When you look at the CCC statement again you see that it says that purgatory is entirely different than hell. I agree, and I stated the ways it is entirely different. But would you go as far as to take that as an absolute statement that every single aspect of purgatory is entirely different? In purgatory and in hell, the Church has already established that part of the punishment is deprivation of the beatific vision and is fire. I have explained that these are both entirely different in that the deprivation is temporal verses eternal and the fire effects the souls differently. You are choosing to interpret the CCC passage further, however, than it needs to be as to ascribe the fires themselves as different matter, which the passage neither requires nor implies. Unless you can prove that the CCC is saying the fires are of different matter in every aspect, then I think the safest bet would be to stick with tradition. The CCC calls the fire a cleansing fire. That is the way the fires are different: in their effect because the fires in hell cannot have such an effect on those souls whereas the fires in purgatory do. Make sense?
I am unaware of where St. Thomas ever positively taught contrary to the Immaculate Conception. Please offer supporting evidence that he positively taught such or I ask you retract your allegation. Here is an article that argues the contrary:
the-pope.com/stThomas.html
Let me again restate the magisterial authority given to the writings of angelic doctor of the Church:
From the encyclical Aeterni Patris by Pope Leo XIII:
“Innocent VI: “
His [Aquinas’] teaching above that of others, the canonical writings alone excepted, enjoys such a precision of language, an order of matters, a truth of conclusions, that those who hold to it are never found swerving from the path of truth, and he who dare assail it will always be suspected of error.”
22. The ecumenical councils, also, where blossoms the flower of all earthly wisdom, have always been careful to hold Thomas Aquinas in singular honor. In the Councils of Lyons, Vienna, Florence, and the Vatican one might almost say that Thomas took part and presided over the deliberations and decrees of the Fathers…”
papalencyclicals.net/Leo13/l13cph.htm