Now that you mention it - if they were being cleansed of original sin, then why did Christ come? Why were they detained in the Limbo of the Fathers?
You can't have it both ways, mate. If they
died with original sin upon their souls, they could NOT be
justified after death. See your citation from the Council of Florence. What do you even understand justification to mean pre-Gospel (or even post-Gospel)? St John the Baptist died before Our Lord. Did he die with original sin upon his soul? St Joseph? They were detained in Limbo because
no one could enter Heaven until Christ paid the debt
in actu, not just
in potentia - and He, rightly, was to be the first to enter into His own kingdom. Btw, to be without original sin is NOT, strictly speaking, the same as being filled with divine grace.
That you think you may have made some kind of point with the above comment is rather telling.