A lot of countries use criteria in addition to nationals to pick their teams. There's Russia, who sometimes uses events like Euros to finalize placements, and Japan, where you can qualify with a strong GPF performance. Yes, all contenders are at Nationals but if you can narrow that field to the 2 or 3 most likely contenders, it would help the USFSA to have one more data point if it chose to go that route. As for your other hypotheticals, decisions could be made on a case-by-case basis.
We fielded a weak men's team in 2011, and injury issues with our ladies the last two years. The USFSA needs more options to deal with these situations; it couldn't even get Tonya Harding off the team in 1994 because of the rules in place.
I think you and MM place too much emphasis on the "fairness" aspect, which can be at odds with the mission of USFSA. Worlds is the one event which the organization should have a little latitude because the results have future implications.
But those systems have their shortfalls as well. In 2006, Miki Ando was SIXTH at Nationals but was sent because she had the most points under its current system (basically skaters were given skaters based on previous results). But that didn't prevent her meltdown at the Olympics. Also, it's worth noting that they haven't done the GPF thing since 2010.
And though the U.S. fielded a "weak" (or newbie, rather) team in 2011, the top two on that team still finished better, on the whole, (9th for Richard, 11th for Ross = 20) then the team everyone felt should have been sent and was sent this year (Jeremy 8th, Adam, 13 =21)
The mission of the USFSA extends beyond elite skaters, which makes a small percentage of its membership. Although having people win international medals could potentially boost participation, perhaps, in the end, I think it really has little baring over most of USFSA's members.