OMG-
pinseeker. Adler has, repeatedly, quoted the NEW verbiage regarding Final Deviation Check-in options under CBA2015.

I don't need to "get permission" to buy a Deviation ticket that has a Pairing's leg on it to accomplish this Final Check in.
There is no "forgiveness" to request.

Once I've accomplished my final check-in, whether it's as part of a multi-leg DH to get to my trip, or your hypothetical I changed my mind MEM-ATL DH...any Flight Cancellations, reroutes, etc...are on the company and up to the company to decide how they want to fix.

It's up to US to know and Defend the CBA. I have no doubt CRS will make blanket assumptions as to their responsibilities and think every Deviant Travel is solely on their own. But that Onus has Changed once you make the Final Check-in.
IF you never make it to your connecting flight, then you won't have made the Final Check-in and everything is still on you.

TOTALLY different situations.