A source needs to be provided, showing that a forced baptism is a valid Baptism, and that Original Sin is removed and the character received in such circuмstances.
I agree that if it were valid, justification would not take place.
When I say a forced baptism, I mean the attempt to administer baptism to a person who does not consent to such a thing.
In such a circuмstance, I think the person who commits the mortal sin would be the person attempting to administer baptism, and not the person receiving.
If the recipient desires to and receives the valid sacrament outside of the Church, the sacrament does it's part so far as removing Original sin, yet at the same time he commits a mortal sin. This is why conditional baptism is not an absolute requirement at all times for converts to the faith who've been baptized before entering the Church.
If one receives the sacrament for inheritance, marriage or some other reason without having a desire to receive it, I do not see that as being any different really than situation above.
Infants have their God Parents do the desiring for them. Trent's catechism states that the Church decides for insane adults, but those who never expressed a desire to be baptized are not to be baptized at all, except in danger of death.
Trent's Catechism.......
The faithful are also to be instructed in the necessary dispositions for Baptism. In the first place they must desire
and intend to receive it; for as in Baptism we all die to sin and resolve to live a new life, it is fit that it be
administered to those only who receive it of their own free will and accord; it is to be forced upon none. Hence
we learn from holy tradition that it has been the invariable practice to administer Baptism to no individual
without previously asking him if he be willing to receive it. This disposition even infants are presumed to have,
since the will of the Church, which promises for them, cannot be mistaken.
Insane, delirious persons who were once of sound mind and afterwards became deranged, having in their
present state no wish to be baptised, are not to be admitted to Baptism, unless in danger of death. In such cases,
if previous to insanity they give intimation of a wish to be baptised, the Sacrament is to be administered;
without such indication previously given it is not to be administered. The same rule is to be followed with
regard to persons who are unconscious.
But if they (the insane) never enjoyed the use of reason, the authority and practice of the Church decide that
they are to be baptised in the faith of the Church, just as children are baptised before they come to the use of
reason.