OK, then go Fellay-ite. If we are forced to reject it and to establish an apostolate over and against the official hierarchy, then it must indeed be grave. But if it's not that grave, as you now claim, then there's no excuse for severing communion with and submission to the hierarchy. Then begin the task of resolving said ambiguities with the "hermeneutic of continuity" and return to the fold.
I, on the other hand, submit that these errrors ARE that grave; there's a fundamental shift to a subjectivist ecclesiology.
It can be argued that V2 was not grave since there was no obligation. It's errors can lead to grave heresies, but technically, they do not. This is the diabolical disorientation and cleverness of satan! The heretical interpretation of V2, however, is imposed as an obligation BY THE BISHOPS and therefore the matter is grave. This is the 'emergency situation' spoken of in canon law which allows priests to provide the sacraments.
The second matter which is grave is related to the new mass, which OFFICIALLY is NOT obligatory, but again, THE BISHOPS say otherwise, in contradiction to Pope Paul, JPII and Benedict and in violation of Quo Primum. Such an obligation FROM THE BISHOPS to attend the indult and accept the new mass is THE ISSUE, and we cannot be forced to commit sin and attend the illicit, probably invalid, and definitely immoral new mass.
If the issue is only V2, and there was no new mass, then yes, we could all work with the 'hermeneutic of continuity' experiment. But the issue of the mass IS KEY. The new mass is illegal and it is illegally imposed by THE BISHOPS so we must resist and we can because the law is on our side.
+Fellay and company don't take a hard stance on the new mass, which is why their version of R&R is illogical and destructive. The mass is the life of the faith; V2's importance to the enemies of Christ is nothing compared to the new mass - which is THE MAIN FORCE of destruction.