Alvin can leave, unless he truly has developed attachment to the house beyond tabby. If Tabby fears the commitment/rat-race so such as to run, leaving Alvin to inherit the same very commitment/country-rat-race that he runs from…. then leave. Let the house, the garden, the inheritance go as granny directed…to the cats.
Alvin has nothing at all to lose, as you can’t lose what was never yours. Yes, can lose the opportunity to inherit something he didn’t really want, something that Tabby is willing to give up unless he pawns his burden upon Alvin…so if Tabby wants to leave, then start packing your own stuff. Let him know that without him…there’s nothing keeping you there. Go back to your life having gained better perspective on what you want, when someone is willing to share that dream.