I think Joe and Heather are first alternates. They were in the draw with 15 seconds to go before sign in closed but Harri Heliovaara signed in with Ulrikke Eikeri at the last moment not leaving anyone time to change partners.
The cut-off for Olympics mixed doubles will be really high with only 16 pairs. That said, the ITF is limiting entries to one (rather than two) per country which will give more opportunities.
That's a classy win. Heather is a really good mixed doubles player. She's a good doubles player period, but isn't the kind of player to be able to carry a weak partner, eg Lucia Bronzetti.
Making the most of my month's subs to Discovery+ and have just watched Joe and Heather's match. Very entertaining and they looked as though they were having a great time!