I guess what I would recommend is that jurisdictions be clearly defined, and that the World, Regions and Municipalities all receive their own exclusive jurisdictions and that this be respected forever by the central World government.
Some regions might want to take on extra jurisdictions, and there should be a formula for this - though there would be some jurisdictions that would remain permanently with the World government (ecology, force, science, education standards) and others that would remain permanently exclusive to the regions (language, culture, education delivery, health delivery), others exclusive to municipalities (transportation, certain types of taxation, architecture and urbanism).
Obviously, the selection of which jurisdictions would go to which layer of government would be a necessarily long and thorough debate.
I agree with everything you've said.
Regarding culture, while in the short run the (con)federation would maintain cultural independence, the end goal of course would be to reduce all cultures to a single "human" culture- everyone sharing the same language, style and apparel. I would see no need to rush that process of course, as I would assume that such a reduction would occur inevitably over time.
As for "education delivery [&] health delivery"-- the former can be considered cultural, and I would not object to the regions dictating the exact areas of study and/or curriculums (curriculi?), at least until the arrival of the singular "human" culture.
Healthcare/social security on the other hand is a trickier issue. I certainly believe that given economic disparity some regions will require greater intensities of welfare then others, but again the end goal is singular. One welfare system for all the people of Earth.
The real issue, I think, is correcting the damage years of disparity between nations has created. That damage should be "repaired" with as much haste as possible.
For that reason, we should try to establish some short term and some long term goals for the EG. As you've suggested, in the short term the EG should protect cultural independence, while working furiously to establish a status-quo of wealth between member regions. This may require great diversity of approaches- different applications of welfare for different regions.
In the longer term, the EG should be prepared for the arrival of a singulars: universal welfare systems and universal culture.
this is just another word to get hung up on.
We definitely need to settle on some clear and exact terminology to reduce confusion.