So are you saying that if people go to these masses for that reason they are sinning, or mortal sinning? Do you believe the priests doing these masses are mortal sinning?
There is a difference between objective mortal sin and subjective guilt for it. For example, if a 3 year old blasphemes, this act is wrong and offensive to God. Objectively, it is a sin because blasphemy against God is evil. But subjectively, the child is not guilty since he doesn't know what he is doing. Just because the child is not guilty does not mean that God is not offended, nor does it mean that the act is not evil.
In the same way, objectively the TLM is true, good and beautiful. The NOM is immoral because it's illegal, it's sinful if it's invalid and, in many cases, it is sacrilegious, absolutely blasphemous and odious in God's sight. Further, the indult is objectively wrong because one who attends is publicly accepting the NOM as good, when it is not. An indult mass is also said in the same church where the NOM is said, and due to its sacrileges and blasphemies, such churches are desecrated and according to canon law, a mass is not allowed to be offered in a desecrated place because all that is given to God, and even the church where it is given, must be pure and spotless. If it is defiled, then God is offended because He is given less than what we are able to give, like Cain who was not pleasing to God since he didn't give his best fruits and animals.
If the english catholics would rather suffer martyrdom instead of going to a mass where the priest had accepted Henry VIII as head of the church, how much more should we avoid the indult, which is an even greater compromise? In the case of the english, at least for the first few years of anglicanism, the mass was EXACTLY the same as it had been before. The priests were all valid, the mass was valid - the only difference was the "condition" that those who attend are publicly accepting Henry VIII as their pope.
The indult has a condition as well, which is much worse than anglicanism. Those who accept the NOM as the "ordinary form" of the Church, as being pleasing to God, are accepting all the communion in the hands, all the talking, hugging and irreverences, all the faulty consecrations, all the eucharistic ministers, etc, etc, etc. All of this is accepted because it's ALL PART of the NOM.
From a subjective, personal guilt standpoint, only God knows who is guilty and to what extent, for those that attend the NOM or the indult. All we can do is eduate people about the objective evils of the NOM and indult and let God deal with the heart and with the level of sin.