If one is starting out on a BA flight, such as at MRU, and one wants to purchase the ticket at MRU for price reasons, is it a requirement to do the initial routing and MRU-based pricing through BA?

Or could one do the routing and MRU-based pricing through another OW partner (say AA), and simply show up at MRU (the appropriate 21 days in advance) to purchase the ticket there, thus only dealing BA-MRU at that time? Just wondering if there are ways around working with BA-MRU if a problem, whether technical or otherwise, does seem to continue?