For the record, my speculation was that MundoFox would replace the 10.2 Fox feed, and that putting MundoFox on 45.1 would be even more interesting.
Having said that, Phoenix isn't "most of the country". Here, Spanish-speaking people come from all walks of life and income strata. But even if Phoenix were average, and 30% of Spanish speakers don't have cable, that means that 70% do. Since the Phoenix metro area, currently about 4.5 million people, is about 40% Hispanic, that's 1.8 million people, or 1.25 million cable subscribers, the majority of whom speak Spanish, and most of them from Mexico or Central or South America. And I think that here, cable/satellite subscription rates among Hispanics is pretty close to that of the general population. The Spanish-language television market here is lucrative - just ask Univision, which boasts the #1 newscast in the market, and recently, the #1 station with adults, regardless of language. Meanwhile, ever since losing Phoenix Suns basketball, channel 45's schedule has been moribund. So yeah, it's plausible that even sharing revenues with a Columbian broadcaster, MundoFox could generate higher revenues for Fox than MyNetwork does now, provided that Fox could put quality programming on the channel to rival Univision, something that the Telemundo O&O has so far failed to do.
Beside that, your statement that "O&O's have some leverage to get subchannels on cable" tears apart your argument that Fox wouldn't put MundoFox on a .1 subchannel. With the influence you say they have, they should have no trouble getting MyNetwork on cable as a .2 sub. So then it becomes a question of who gets the "prestigious" .1 slot, and in Phoenix, it's a very good possibility that it could be MundoFox.