this is the only way officially.

As mentioned, you can call for E+ - agents have typically done this (though I've never technically seen indication in writing on whether E+ For companions on separate itineraries are or are not allowed, but it seems to be standard in practice). Bags would have to go on your record, and UA will likely allow your companion to Board with you/check in at the Premier line if you tell them they are with you - same at security - though they wouldn't technically have premier access.