Dimezone was developed to avoid the problem of hydrolysis that occurs with Phenidone in alkaline solution. Unfortunately Dimezone turned out to be less soluble in water than Phenidone. To correct this the structure of Dimezone was altered slightly to make Dimezone-S (S for more soluble). Google the three structures to see what was done. At the pH of the developer you are working on there would be no problem with hydrolysis. So there is no reason that you cannot use Phenidone. I have never heard anyone other than yourself say that Phenidone produces more fog than Dimezone-S. A small amount of fog is always produced during development and really cannot be avoided. If you attempt to eliminate all fog produced by a developer with bromode or benzotriazole the activity of the developer is serverely compromised.
As far as activity there does seem to be less activity with the two Dimezones as compared to Phenidone that cannot be explained by the slight increase in molecular weights. So replacing one developing agent for the other is problematic in this respect. Perhaps this accounts for the increase in fog that you observed when Phenidone was used. You were simply using more Phenidone in relation to Dimezone-S.
Yeats ago there was a developer called Huebl paste which was produced by grinding Glycin, sodum sulfiie, and potassium carbonate together in a mortar. A portion of this paste was then dissolved in water to make a developing solution. (Huebl is an alternative spelling of the man's name when there is no umlaut U in the type set)