Mileage, as always, is determined by the fare class of the operating carrier. So mileage for the AC segments are going to be determined by the fare class on the AC flight.

To get that, as others said, is tricky. Assuming this is a codeshare flight (I.e. Booked as UA flight XXXX operated by Air Canada, and not as AC flight YYY) looking at the AC site won't matter - it is still showing the codeshare information and using the same fare class - even if it doesn't map to the same, it will still show the UA class, and use that in its translation to Tango, Flex, etc. even if it's actually not the case for the operating fare class.

That said, K, is a pretty low fare class and extremely unlikely to be anything other than Tango on AC. The only way to check for sure is to call AC and ask them, making sure you are specific to ask about what it maps to on them (otherwise, they may just say K looking at the codeshare). Then, look for that in the AC table on the MP site. Disclaimer: AC is known for very long hold times - expect a 30 - 60 minute hold time with AC, even on a good travel day with no irrops. EF may also be able to tell you if you have a subscription, except that is the one part of EF I can't properly determine how to understand - maybe others can help.

As for 25% vs. 100%, yes, flights are credited by segment and not as a whole. Believe you should get 100% on international and a portion on the transborder, but again, checking the table on the MP site after getting the native fare class is really the only way to do it.