I agree that it would be stupid to say "Offer the most expensive options first."

But I think that your interpretation of "Avoid rebooking on Delta and other interline airlines...." reads plenty into United's own guidance to arrive at the suggestion that "We have the agreement with Delta, but really try to use the lower cost options first..."

There have been many reports in this forum of pax being told that rebooking on OALs during IRROPS is not possible. I don't think people are making this up, and I think such a denial that interline alternatives exist is more consistent with the "Avoid rebooking on Delta and other interline airlines" instruction.

I also agree with LarkSFO that FTers who are well informed and equipped can stick up for their preferred interline alternatives and stand a better chance of getting what they want after much agitation. But this turns what could be a 'flyer friendly' IRROPS moment of UA helping its customer out of a mess into another episode of UA grudgingly giving a remedy for their own under-performance in maintenance and crew scheduling.

Maybe it is sub-optimal to have an interline agreement in place and then discourage using it to the point that customers walk away angry at UA's customer care even after the expense of an interline rebooking has been incurred?