It would actually be the other way around. 1) The main timeline games happen up to Sonic Forces as normal. 2) Infinite obtains the Phantom Ruby, weird stuff starts happening. The Ruby appears on Angel Island in the past and thus causes a divergence. 3) Sonic Mania happens in the past: at this point as a divergence this Classic Sonic is now from another dimension and not the same past as before. 4) At the end of Mania the Ruby transports Classic Sonic to Forces. 5) Forces happens. Classic Sonic goes home. 6) Sonia Mania Plus Encore Mode happens. As a side note, if Generations Classic Sonic were to be pulled to the main timeline future again (bearing in mind his future is now Sonic 4 and separate) then he would be from another dimension.