Those priests have never been a part of the RCC, and their formation took place entirely outside the RCC, so being admitted to the RCC is not like returning home in their mind.
That’s not right, they are part of the Catholic Church.
They rule their seminaries yes, but this is common practice in almost every single religious order.
Their loyalties are primarily to the SSPX, and they would not support any reunification scheme that does not allow for the continued existence and a high degree of independence for the order.
Again, not true.
Their loyalty is to the Church and to the deposit of the Faith, not to themselves.
And about a “treaty” to bring the order into full communion, we should check their own sources and avoid speculation, for they and the Roman authorities know more than we do.
Another issue is that they have accumulated a substantial population of the faithful that associate with the SSPX and who have essentially cut off all ties with the RCC. The OP’s daughter’s perspective in-laws seem to be among them.
And you have Franciscan, Benedictine, Dominican…faithful that won’t go to other parishes nor seek other priests outside their favorite order. The OP’s daughter is a case involving two people out of thousands around the world, so it should not be used as reference.
I’ve seen SSPX faithful being quite ok and welcoming, not chastising those that attend NO masses or those that go to other Tridentine masses.
Besides this, the priests do interact with other diocesan priests and/or members of religious orders, two good examples are the “borrowing” of parishes during pilgrimages and the announcement that the Capucin friars from San Giovanni Rotondo have given one of Padre Pio’s gloves to the SSPX South American District. I doubt that Church authorities would do the same to some schismatic, crazy group.
In other words, the SSPX has taken on a life of its own outside the RCC. As time goes on, they will become even less interested in “reuniting” with Rome, as more and more clergy and faithful no longer see it as a matter of “reuniting”.
Again, the SSPX is not outside the Catholic Church, otherwise the Popes wouldn’t bother to talk to them, nor would Church authorities care about them, nor would a Swiss bishop choose their house for his retirement.