It should be bookable as long as it's on same flight number. However, AAExpDFW is right, you might need to be confident, the agents might need to do it manually. In Sabre, it will show up as two segments, but on a ticket it will show and use only one segment. Amadeus usually shows those as single segment.

I used multiple of through flight numbers with my previous RTWs to minimize the segment usage. Even booked it other way, AA109 LHR-HNL as one segment. The ticket was issued by QF. And my previous AA ticketed xONEx included also some through flights. Many of those showed up in EF as a connection, but was sold as one segment. IME if there is different equipment on the legs, it will show up as connection in EF, if it's same equipment code then it will mostly show up as through flight.

Theoretically yes, but I guess rating desk would deny the routing DFW AA DFW. Mileage is based on the direct distance between ticketed points, stops on the coupon are not taken account. That would give 0 miles if it would be calculated based on the distance.