BYU just needed a place for their other sports. Trying to lure SDSU was the brainchild of K B and the USU Pres.
Yes, BYU did try and seek assurances that the WAC schools inviting BYU into their conference wouldn't turn around and bolt after BYU joined (and thus truly hurt BYU), which they did anyway.
The old MWC schools were just butt-hurt that BYU (their cash-cow) left. Have to laugh at the fact that without BYU the MWC TV immediately folded and the MWC had to drop a few sports.