After reading the above, I'm confused whether you're talking about:
1) Air Canada codeshares on UA Metal
or
2) UA codeshares on AC Metal

It makes a huge difference.

Case 1) It shouldn't matter at all what class you book an AC codeshare on UA metal - UA flights with UA paint on them and UA crew inside accumulate based on UA's rules for earning on UA. In fact, when you check in with UA, even on a codeshare, your UA boarding pass will show a UA flight number, even though United.com showed the codeshare flight#. Also, your booking class will be whatever the UNITED booking class is for the AC (or US, or LH or ...) booking class maps out to, which may well not be the same letter. Often either the codeshare airline's reservations staff, or UA's, can tell you what the UA booking class will be. UA does not have any UA REVENUE booking classes that do not earn UA miles.

Once in a while this goes wrong, and the booking class "mapping" doesn't work. I had a US codeshare/UA metal flight in US "G" class that should have mapped to something like S or E on UA, which earn UA miles. However it stayed as "G" on UA, and "G" is not a UA REVENUE class (it's a travel industry class) so I didn't get miles, even though US "G" ("real G") does earn UA miles. Took a call or two to UA MP to fix it.

Case 2) This is where the UA booking class for the UA code doesn't make a difference - it all comes down to what the operating carrier's booking class is.


The start of your post talks about a UA codeshare ON AC metal, booked in UA W. UA W is a miles-earning higher-than-cheapest UA class, which appears to have mapped to AC's miles-earning higher-than-cheapest AC "A" class. You don't need to avoid UA codeshares booked in UA W, you need to avoid AC flight#s booked into AC W. UA W <> AC W.

The end of your post talks about booking a UA-metal flight via an AC codeshare# to get the 500-mile minimum. That's wrong on two counts:
a) UA posting goes by the metal, not by the code, it will post as a UA flight#, UA booking class, UA rules.
b) UA no longer gives 500 minimum miles on any carrier. It doesn't matter what AC does with their own program, nor what an outdated united.com page might say. 500-mile minimum is gone from the Mileage Plus Program, not just from UA flights.

I think I answered your questions but I'm still confused if that's really what you were asking.

In general, with a very few exceptions mostly on the newest *A partners like Turkish and Shanghai and Air China, and a couple of other partners, in UA MP and most other *A programs, the codeshare does not matter - what matters is the operating carrier and flights post as the operating carrier. This is very different than how oneworld programs work, such as the wacky AA rules that do or don't allow credit or EQP or other items on the same flight, depending on whether or not AA is the code. With UA, it's (almost) always the Metal that counts.