It's unconstitutional to count votes received after Election Day.

Quibble - the ruling didn't hold it was unconstitutional to count votes received after Election Day. It held that doing so violated Congressional statute.

In sane times, there would be neither conflict nor confusion over this Very. Simple. Concept.

Why? There's lots of circumstances where we consider the date upon which something has happened as being the date of mailing, rather than the date of receipt. That doctrine is called the "mailbox rule" - we had it drilled into us a lot in first year law school. The opinion alludes to it, but dismisses it out of hand by noting that Congress didn't specify a mailbox rule for when "voting" happens for the purpose of a federal Election Day. But neither did they say it was prohibited - and in a federal pre-emption case, usually you need to find a conflict with what Congress actually said, especially when states are given the right to specify their own rules unless Congress provides otherwise.

If a State wants to say that the date your vote is cast is when you put in the mailbox, just like your tax return is filed the day you put it in the mailbox (and not when it is received), that seems a reasonable construction of the federal Election Day statute - and one that should not be prohibited unless Congress has specifically acted.