Hey ... it's a power trip.
Apparently Father Tran hasn't learned that yet.
It seems as if there are two issues here:
1. Obedience - this is non-negotiable, unless the priest and bishop are directing them to stand during the consecration. The only place where kneeling is absolutely required is from the beginning of the eucharistic prayer until the great "Amen". If the bishop/priest are saying, "you must stand after the 'Lamb of God' and continuing through Communion", then I'm afraid they are within their rights to do so. In which case, disobedience to the directive of the bishop would, indeed, be a grave sin.
2. The Hammer of Tran - Tran and the bishop's directive to expel the disobedient from the diocese may be within their right, but the level of hypocrisy is breathtaking when one considers their reticence to do anything about high-profile Orange County politicians and others who work to support the "right" to abortion.
Thus, my feeling on the matter is this:
1. The parishioners of this parish need to be obedient to the bishop. No ifs, ands, or buts. They are fostering disunity by their public disobedience. Whatever the lack of merit in this directive, the first duty - as long as the Bishop is within his rights - is to obey the bishop. The whole history of the Old Testament is couched in the consequences of disobedience - from the Garden of Eden to the Hebrews in the desert to Moses himself to David, etc. etc. etc.
2. Fr. Tran is wielding way too big a stick on this. Just my opinion, but he's not exhibiting pastoral care by throwing 80 year-old ladies into the street. And given the bishops disinterest in doing anything about excommunicating pro-choice politicians in his diocese, I wouldn't want to be in his shoes on Judgment Day if this is the hill he wants to battle for.
3. In the end, Christ knows these people want to kneel in reverence. But disobedience is greatly offensive to God. The way the Church is set up, the bishop has the final word on local issues unless he is flat-out violating canon law, in which case, he would not be in communion with the Church. In this case, it appears he is not violating canon law, so, parishioners can either go along with it, or go to a new parish/diocese. Not one of these parishioners will be held accountable for being obedient. If there's any accounting for, it's going to be the bishop and Fr. Tran. But since we're not in the business of passing judgment, the good people of that parish are going to have to entrust it to God and know that He knows their hearts.