I received online support from a Google representative yesterday and the problem has now been resolved. Although we don't know the reason this happened, my google account was no longer linked to my google wifi although fortunately my wife's account was (and was now showing as "primary"). When my wife sent me the invitation to join the home (as described above), it couldn't be added to my google account because I already had 5 homes linked to my google account. To resolve this, I had to first delete the home in Google home so when the invitation was sent again, it could be added - the Wifi icon is back in both the Home and Google Wifi apps! There was no explanation provided for why this happened. When I started the support session, the rep asked if I had done a factory reset on my wifi puck and I said no because I at least had the ability to manage my network on one account and I didn't want to risk losing access on both accounts - he concurred. I did find some information from Google that I suspect played a role in this - and this is total speculation - but the notification said that if your Google wifi puck was not connected to the internet for 6 months, it would be automatically and permanently deleted by Google. I suspect something in the migration process from Google Wifi to Google Home was responsible for my Google Wifi account being de-linked from my google account.
