USSSA follows USSSA rules, which just like all other levels of play they allow national rosters.

SSUSA has their own guidelines which they have had for a long time (Same with ISA, SPA, etc.). In SSUSA, your roster must be from bordering states, with an exception for 2 players that can be from non-bordering states but the same side of the Mississippi River, as the state the team is registered out of.

Yes, National USSSA senior teams will either adjust their roster to play ISA,SPA, or SSUSA (which I believe draws the most senior teams overall), or not play. Their decision.

The talent on the teams at SSUSA worlds is second to none. FBI/Miken/Worth 50 Major+ team can attest to that. They were probably the best 50+ team all year long, and had an off weekend by their standards at SSUSA worlds. The teams there were all very good.