Imposition of "conditions", any conditions no matter how generous, implies that it is the SSPX which has strayed and is on the outer and that the Church is playing the role of father welcoming home a prodigal son. Right there, you have a big red flag for the SSPX hierarchy and many of its followers for it's clear that they think of themselves as the true guardians of tradition and not the Pope or the Church. Ergo, the SSPX has no reason to sign an ultimatum. Rather it is they who should be imposing conditions.
If you don't believe me read this page from their website. A casual perusal of this material seems to clearly indicate that conditions #2 and #3 written above, will not be accepted.
I'll wager $20 with any takers that this ends badly.
I agree that we need a certain amount of their influence, although the unfortunate thing, as we discussed above, is that their isolation from the Church for so many years has in some cases (not all, by any means) led to the development of some peculiar attitudes that we don’t particularly need.
Personally, I think this will probably lead to a split within the SSPX. Many of them do want to return in good standing, but there’s a certain nutty hard core that probably wants to cling to its specialness and won’t come back.
I’m glad the Pope is giving them a specific date (which is almost upon us), because they have to make a decision one way or the other: whenever they have been given time in the past, their various factions have used it to prevent anything from being done.