The other three SSPX bishops do not seem to be in favour of an agreement with Rome. A French website claims that 98% of the SSPX is against a deal. If this is true then it does seem that Bishop Fellay has enormous power as Superior-General and can ride roughshod over majority views and opinions.
If this is true, then Bishop Fellay could only do this for so long before he finds himself to be a king with no kingdom. Being a Superior-General, again, is a political title. Yes, people swear allegiance to him but as in all things they belong to God first. I think rather that Bishop Fellay, as ambitious as he is said to be, can't be so foolish as to lead where no one will follow.