OK, there is a lot to unpack there. I agree with a lot of the things you said, but unfortunately you have one major fact wrong. Let me begin with Utah State joining the Sun Belt Conference and I'll address the WAC situation later.
Division I-A (FBS) conferences are required to have eight full-time members. This rule was implemented in 2005, but was known about as early as 2002. In 2002, the Sun Belt Conference had seven members:
1. North Texas 2. New Mexico State 3. Arkansas State 4. Louisiana-Monroe 5. Louisiana-Lafayette 6. Middle Tennessee and 7. Idaho.
But as you note, Idaho and Louisiana-Monroe were "football-only" members of the conference. So technically the Sun Belt only had five full-time members. That means the Sun Belt needed to add at least one school and get some others to sign up as all sports members.
This is why the Sun Belt Conference needed Utah State. In order for the conference to survive they had to expand. In 2002, the only expansion options were independent teams. Utah State and Troy State were those options. Troy State accepted an invitation in 2003.