Regaring the distinction between the state of mortal sin, and commision of mortal sin, please see #4 here. If Baltimore Catechism defines mortal sin broader so that it does not require deliberation, I'd like to see Baltimore's definition; otherwise, we have to conclude that the reference to the state of mortal sin in the context of the Limbo is in contradiction to what mortal sin is, which probably explains why it is not in the current catechism.
The particular judgment and the final judgment of the soul are identical.
the designation of Limbo as a province of Hell seems arbitrary.
On the contrary, it is the teaching of the Church. To designate it as part of heaven is a condemned error:
Can. 3. It has been decided likewise that if anyone says ... that it might be understood that in the kingdom of heaven there will be some middle place or some place anywhere where the blessed infants live who departed from this life without baptism ... let him be anathema. (Council of Carthage 418/Mileum 416)
Thus the Church teaches:
The souls of those who die in mortal sin or with original sin only, however, immediately descend into hell, yet to be punished with different punishments. (Council of Lyons II, Variant Readings, Profession of Faith of Michael Palaeologus)
It teaches ... that the souls ... of those who die in mortal sin, or with only original sin descend immediately into hell; however, to be punished with different penalties and in different places. (John XXII, Letter to the Armenians)
Moreover, the souls of those who depart in actual mortal sin or in original sin only, descend immediately into hell but to undergo punishments of different kinds. (Council of Florence, Decree on Reunion with the Greeks)
As for the difference over the state of mortal sin, suffice it to say that it is a difference over terminology, nothing more. All men inherit sin, which is the death of the soul, from Adam. To call it mortal, in the sense of 'deadly', is quite correct, so long as 'mortal sin' is not defined as 'actual mortal sin'. See Q. 18-20 of St. Peter Mohila's Confession here.
There is nothing speculative about it at all. The saved are in Heaven, the damned are in Hell. Heaven is glorification and communion with God. Hell is seperation from God and punishment. Those guilty of actual sin are tormented with the punishment of eternal death, the worm of conscience, and eternal fire.
If you are taking as speculative what I said about these not being seperate places, again, there is nothing speculative about it. Scripture is perfectly clear that the saved and the damned see each other (i.e. Lazarus and the Rich Man) and are both before God. Their experience of this is all that seperates them, the saved rejoice in love, the damned are turned inwards into their own magnified perversions and hatred.
Since the souls in Limbo do not suffer, and have exceptional ways granted by Christ to reach the supernatural happiness of the Beatific Vision, the designation of Limbo as a province of Hell seems arbitrary.
Where do you find a teaching about "exceptional ways" to "supernatural happiness"?
If Limbo is not part of hell, then it is part of heaven, and those with original sin are already saved, or rather, original sin never was a problem for us requring a Redeemer. This brings us squarely back to Pelagianism. The whole concept is nothing but a trap.