USFSA & CoP
It makes sense to have Nationals judged by the same system as Worlds. It would be quite silly for USFSA to judge Nationals using ordinals and then send skaters to CoP-judged Worlds. It would put US skaters at a disadvantage, since programs should be created differently for the two systems.
Third spot on US team
Once again, I fully realize that USFSA, like any other federations, has the authority to send whomever they chose. All I was pointing out is that so far noone has actually given me a concrete example of when they chose to do so. The only way I can see USFSA do this is if a major skater is out due to injury (such as they could have sent Tim instead of Matt had Tim been OK to skate).
In other federations, it's different (Russia sent Irina & Vika, France did not send Stanick; Japan announced pre-Nationals they were sending Fumie and Shizuka no matter what, etc.).