In these scenarios, these are the challenges:

Book a multi city flight set and fail to show at one, the remaining segments are cancelled and the ticket remaining value may be lost.

What is legitimate:buying a repositioning flight. E.g. say it’s substantially cheaper to fly AUS-LHR (via DFW) than it is to fly DFW-LHR, buying a flight DFW-AUS one way or return or driving your car and parking at Austin Bergstrom instead of DFW are fine.

Buying nested tickets is OK, too - as long as they’re not purchased to bypass length of stay restrictions. Buying two nested low price (1) DFW-YVR-DFW and 2) YVR-DFW-YVR) tickets and using the coupons to violate Saturday night stay requirements will likely violate the Cinditions of Carriage, and yes, AA can become aware and has complete control of your AAdvantage account.

See FT: FAQ: Skipping Segment, Hidden City / Point Beyond / Throw Away Ticketing (master thd)

and AA CofC, Ticket Validity