There is no ground for the FTC to block the merger. There has to be a legal argument that the chain would be anti-competitive and there isn't one like there is with the big tech companies that the FTC is going after. Whether those arguments against the tech companies are strong enough to be legally valid is beyond the purview of a forum like this, but they are easy to understand and follow with a couple minutes research.
What argument could they put forth about this being anti-competitive? There are the three well documented markets with arguable competitive crossover, but those markets are still saturated with other competitors. No argument there to block the whole merge. Maybe to divest one of the properties, but with all the other nearby competition that would be a tough argument to make. I think the "nightmare" scenario for the new chain is that they have to pick one each of SFMM/Knotts, SFA/KD, and SFGA/Dorney and sell off the other and I would call even that unlikely.
There is nothing to work with in terms of blocking the whole merger.