Unity is a requirement of the Church, and any intentionally actions to create disunity are forbidden. Having read some comments about the Franciscan case, and now this case, a concern mentioned was that those offering the Tridentine Mass were using the form as a sort of protest, almost a passive-aggressive statement. If, and a big if, that is the case, it would make sense for the local ordinary to forbid it use, by the “offending” parties, though not for the diocese as a whole.
Let’s stick to the case in question.
Regarding Vatican II, are you a sedevacantist, or do you just disagree with its outcomes?
If you are a sedevacantist, there is no point in this discussion. If you aren’t, then your questions about unity in Vatican II are not relevant in this case.
Either way, the issue under consideration is one college and one bishop.