Well, outside of changing Indiana to Illinois and using it for France, I have "done" nothing. So, I'll defer to your workset.
We can probably make about sixteen changes total considering the small Areas in the USA that could be readily combined (IMO).
As to your list, not sure Montana or Quebec need another Area.
The Antilles and the Carolines are both island groups, so I need to review their actual lay on the map. Could split them.
Cameroonian Jungle. I don't think that needs split. Others may campaign there more often than I.
Texas. Could be split, but not convinced it is necessary for gameplay.
Might be able to address almost all the rest, though I would suggest referring to old maps to get some period names (which you may already do).
Don't really think we should alter the seazones. I only foresee bad side effects from that. Plus, I think we should not tackle Regions at the same time (if ever).
Have to research the other comments (Denniss and rwglaub). Edit. For Denniss, we should simply realign the existing provinces and add one (somehow).
Mecklenburg: Bremen, Wilhelmshafen, Munster, and Hannover.
Some New Area: Kiel, Hamburg, Lübeck.
Rename Hannover to something else: Gottingen, Wolfsburg, Magdeburg, and Rostock.
for rwglaub: In the ID box, the regions Benelux and Belarus display correctly for me. Unless, you mean elsewhere? Please explain further.