ok question 2 and it involves nesting vs back to back ticketing.

I have 2 options

1. SEA-SYR with a SYR-SEA in between. In both cases 1 direction is operated by NW and the other leg by DL; but I'm not lucky enough to have the SEA-SYR entirely operated by one carrier and the SYR-SEA entirely operated by the other. Is this nesting, is it illegal, and is it enforced (enforceable) when there are mixed carriers operating?

2. SEA-SYR and SEA-SYR back to back. Again, mixed carriers. I'm having trouble getting this to ticket (I can book them as separate itineraries but want the protection in case of a misconnect) which is why I ask about option 1.

Point is, I really don't want to do anything clearly illegal. I just want the most protection I can have.