Their domestic voltage is also 100v. I've never thought about it before but it does seem like that might pose some challenges in electrifying their road transportation. 240v is considerably more useful when you are trying to suck down 7kw or more.

Now that you mention it, the electric connection in my home in Tokyo was much worse than the one I have in Calgary. They might have connections of 60 amps [1] or less where I lived (whereas my home in Calgary has a 100 amp connection which can be upgraded to 200 if I paid for it)

[1] Found this here (https://www4.tepco.co.jp/en/customer/guide/moving-e.html), I don't recall what exactly it was where I lived.

It is a problem, because there's limited capacity to move electricity between the systems. You can just plug them together, they have essentially two separate grids.

I think it would be a problem if they had ten tiny grids, but is two decently sized grids that are in roughly the same time zone (so peak solar doesn’t need to go from A to B) a big deal?

It was a huge problem when earthquake stopped many power plants in east area.

It is when a 1500MW nuclear power plant built in area A is essentially useless for anyone in area B.

Yes, you can transform from 50Hz to 60Hz, but the capacity is limited and can't be grown infinitely.