This is very helpful color, thanks. The option to potentially use my Air Canada *A Gold number for the benefits (including lounge access for domestic flights), but potentially switch it at the right time to get the UA mileage credit is helpful - will definitely take a read through that thread. Especially if I'm trying to get my UA Gold organically.
Of course that might be more difficult in today's flying economy (flying much less for business now than I did in previous years), so I'm still going to consider the potential status match to Delta (probably not American, as I don't like that they require a fee to even try the Challenge).