As to rebooking on other airlines, typically they try to rebook on AA/oneworld first, and other airlines as a last resort. They can only rebook though to airlines they have an interline agreement with, so DL wouldn't have been an option as AA and DL no longer interline. Many times the willingness to rebook on other airlines is dependent on the reason for the delay. They are more likely to be willing to rebook to OALs in mechanical situations. In weather situations, not so much.
Sometimes in IRROPs, the choice in rebooking is the flight that will get you there the fastest, but taking the downgrade, or having to wait quite awhile for availability but remain in your originally booked cabin. Unfortunately it sounds like your agent wasn't seeing that the 2nd (and main leg) was actually in business class or possibly just didn't care.