Remember that the FSSP was founded in 1988 by a dozen or so priests of the SSPX, led by Fr. Bisig, who didn't want to accept the consecrations of 4 bishops by Abp Lefebvre, as they thought it would lead to a schism, and they feared the whole SSPX would be excommunicated. They thought the protocol of May 1988 was good enough.
Now, it is very possible that Rome has "lowered" the level of their conditions to an agreement with the SSPX, compared to those presented to +Fellay on June 13 2012. Then the main condition was an outright full acceptance of Vatican II and the actual magisterium.
Maybe now Rome is agreeing with +Fellay who said the "problems" the SSPX has with VII could be discussed and resolved AFTER an agreement. Maybe Rome is also saying: If we recognize the SSPX and give it a status of personal prelature, thus having the SSPX going BACK to Rome, then FSSP should go BACK to the SSPX.
Rome would present this as a practical consequence of an agreement with SSPX because, basically, the FSSP has houses that are always in the same locations than the SSPX. So why have two official and recognized traditional Societies in the same towns? It would be a waste of manpower! Therefore, Rome is saying: Let us merge both Societies, and go back to the situation before June 30, 1988, and have ONE big happy family. The merger would also allow the new Society to sell properties where FSSP and SSPX used to have one each, and use the money to open houses where there was nobody before. This would expand the presence of Tradition in the whole world.
I can very well see +Fellay fall for these arguments. As for FSSP, it might be a bit more difficult to swallow, but they will have the prospect of having their priests ordained by surely validly consecrated bishops. Moreover, maybe Rome will even propose that the FSSP superior would be consecrated by the 3 SSPX bishops, so as to replace +Williamson. With the worlwide expansion caused by the merger, four bishops would not be too great a number to satisfy the needs of the faithful.
With these reasons and proposals, it is possible Rome will succeed to obtain the merger, especially if she makes it the only condition to an agreement with SSPX. If the SSPX accepts the idea of such a merger, Rome could go to FSSP and threaten them with suppression if they don't agree. Rome would say: How dare you put an obstacle to such an agreement with SSPX? How dare you refuse to help heal and close the wound opened in 1988? Besides, the reason you had to leave the SSPX in 1988 does not exist anymore, you have to go back to it!
In other words, even if the rumor is false, this scenario is very plausible, and even probable. Come to think about it, I think this scenario will HAVE TO HAPPEN if the SSPX accepts an agreement with Rome.
And the result of such a merger will of course be a catastrophe for Tradition! Indeed, the conservative elements still belonging to the SSPX would be swamped by all those priests and seminarians and faithful who have been steeped in liberalism since 1988!
Very quickly, with the new assignments that would follow the merger, the SSPX faithful would lose track of many of the priests, and they will never be sure if this or that priest they never saw before is originally from the SSPX or the FSSP. I doubt the former FSSP priests would accept to be conditionally re-ordained. Hence: A big problem!
The advantage of a merger for +Fellay, besides a financial gain coming from the sale of useless properties, would be that the arrival of so many FSSP priests and seminarians and faithful would more than offset the number of those leaving the SSPX to join the "Resistance".
The more I think about it, the more this scenario looks like the thing Rome and +Fellay would want to do, and FSSP would not have much of a choice. They surely would not want to become "vagi" and without jurisdiction and all that...