I don’t know how many Old Calendar groups exist, I never took a tally and your figure surprises me. However Canonical Orthodoxy knows these to be true Orthodox churches if they teach no errors (these people tend to be very perspicacious), their sacraments are very real and the pewpeople usually have no difficulties when visiting the canonical church, which is huge by comparison.
I suppose then, that they would say something like the SSPX (and the multiple smaller sedevacantist bodies) would say about Vatican II. Does that answer your question?
As you probably know, doing commemorations on the wrong day can be problematic. The RC has what are called Holy Days of Obligation, and if one is skipped for a non-important reason, or forgotten, or “substituted” to another convenient day it may be considered a mortal sin! So suddenly these issues take on a dimension of great importance for the very scrupulous, which may be the case here.
One other argument some bishops favoring the Old Calendar have made is that it has the (unintended) effect of reducing the number of Fast days, which is true. From their point of view this unnecessarily endangers the Christian in their spiritual development. If that be the case, it would be a great moral failing for a spiritual leader to advocate the New Calendar. (This could, once again, compare to some conservative Catholic people’s opinion of the New Mass, the adjustment of the Friday Fast to allow substitution or the one hour pre-Eucharistic Fast).
Personally, I think the Orthodox have more than enough fast days any way one counts them.
I do not see the calendar issue as a good enough reason for schism, and it is unfortunate. There are Canonical Orthodox on both calendars, just as there are Eastern Catholic churches on both calendars, it should not have resulted in schism, but thankfully in those cases the small uncanonical groups are just as cautious about theology. In that sense they are truly Orthodox, prayers all around for reconciliation.