It's not true to say that Radical Dreamers is an alternate dimesnion that didn't happan, the mere existence of the record in Chronopolis proves it existed.
The whole point of alternate dimensions is that they have things that happaned differently then your original dimension. Crono isn't revived? Radical Dreamers. Belthasar would probably realize Schala was reincarnated, meaning no TD threat, no need for Project Kid, no Chronopolis. You can cross dimesnions in CC because of Angelus Errare, which is a very special condition, and also because you have Serge, the missing piece.
It probably happaned where San Dorino used to be, because Porre is a nation to the SOUTH and Guardia is a nation to the NORTH. Meaning Grezenbul is around the center of Zenan, hence it's where Dorino is (Or even in Fiona's forest!). Also, I would find it VERY unlikely two nations like Guardia and Porre would consist of one town each. There are probably smaller towns that weren't added to the Map's size and importance to the plot.