Technically speaking you could really substitute any fish you want here with varying degrees of success, but comparable fish to branzino would red snapper, any other type of sea bass, or even flounder.

Champagne vinegar can be substituted for white wine vinegar

There isn't really a stand-in for garlic, but if you wanted to make a pearl onion confit with this same method that would also work well with the other components of the meal.

Broccoli can be substituted for rapini

Yukon Gold potatoes can be used instead of red potatoes, really any dense potato will work.

You can make a granita out of essentially any fruit juice that you want, grapefruit could definitely be a nice option here.