I think AA ZED has answered the question well ......

I'd like to point out a couple of things from my own experience.
When you present yourself to a ticket counter, generally, time permitting, the agent will look at the bilateral agreements.
However, what I have found, 9/10, a check in agent will accept the ticket regardless what agreement you have.
In fairness, in these times, there's money on the ticket, you only board if there is a seat, I doubt if your own carrier will decline the billing via interline, providing
the coupons are billed at face value.

United do not refund based on downgrades. So, if there is a Business Class ZED available on a particular carrier, as an employee, you are
expected to have a ticket issued in Y and C. If it transpires that only Y is available, you offer the Y coupon. There are no interactive refunds.
This is understandable. Given staffing levels these days, which Interline Department is going to request an adjustment from another carrier ........

I have found that boarding priorities are very debatable. For example, an alliance carrier may on ' paper ' stipulate that on ' paper ' alliance
carriers are boarded immediately after their own staff, I have found this to be inconsistent where said airline is handled by a Vendor.