Additional question... is it required to use far fa-calendar-alt, or can we just use fa-calendar-alt by itself? Will there be deprecation issues down the line?
If you specify fas/far/fal explicitly, then you're guaranteed to have that particular weight. Otherwise, the weight may depend on the style's choice (though in the ACP, that's less relevant).
The add-on icon might be a bug, though I think you can workaround that by putting the modifier type at the end. Saying that, though it is just a "W" icon, using a brand icon to represent your product (which isn't by that brand) probably isn't ideal, so you may wish to reconsider your icon here.
@Mike ... Any chance of 2 icon definitions in the addon.json file?
One for XF 2.0 and one for XF 2.1. I'm finding many of the "free" font awesome icons have been renamed in the Pro font awsome system, so they don't work for both XF 2.0 and XF 2.1.
For instance fa-television is now fa-tv
Perhaps something like
"icon": "fa-television",
"icon1": "fa-tv"