Key issue in these discussions is to remember the IATA and DOT rules refer to the standard bag allowance for the tickets (this is typically based on the fare class / cabin). Elite privileges are a separate issue and may or may be granted - not guaranteed by the IATA or DOT. That is at the discretion of the check-in carrier.

So when checking in with UA you will get the better of the MSC/DOT rules and/or your UA elite benefits.

With checking-in with a *A carrier you will get the MSC/DOT rules plus your *A elite benefit. (need to check with the carrier to determine if this is weigh or bag count addition and if the bag count is just 50 lbs or higher).

When checking-in with a non-*A carrier you will get the MSC/DOT rules -- there will be no elite benefits adder in most cases.