In your document, go to the first Heading 1 entry, select the numbering drop down on the Home bar and choose "Set Numbering Value". Select 1.
Now right-click on the Heading 1 style quick selector and chose the first option that should say "Update Heading 1 to Match Selection".
I think that should do it. You are right though, this has got hellishly more complex!! There doesn't even seem to be a standard style template that uses numbered chapters!