There is a difference between the erection of the SSPX and the erection of these societies by +Fellay. The prelate from whom ABL acquired approval had jurisdiction. +F has no jurisdiction. Nor, as a bishop, is he properly the S.G. of the SSPX in the first place since ABL always wanted a PRIEST and not a BISHOP to be the Superior of the Society of St. Pius X.
Arguing within the letter of the law (as you do) the permission to function as SSPX ceased in 1976 without the Roman suppression - the approval was only for a 6 year experiment and the experiment ran out in 1976.
Arguing from the Salus animarum
perspective - necessity trumps the law and the law says that itself.
As to the erection of societies - the Archbishop did that as well - think of the Sisters of the Society, the oblate Sisters, the Brothers, the Carmelites etc.
If you follow the "Law" position then the Archbishop only had permission for the SSPX. The SSPX ceased to exist in '76 but evidently it continues to this day.