In my opinion it would have been her best shot to do well at the trials and at world's. A sub 2 runner and a 4:04 1500m runner. Anyone with those credentials has a shot at gold in the 800m. How many women Olympic/World Champions have won gold in the 800m and have a 1500m PR faster than Mary Cain's? The last 4 Olympic/World Championship winners were:
2008 winner: Pamela Jelimo; 4:07 PR
2009 winner: Caster Semenya; 4:08 PR
2011 winner: Mariya Savinova; 4:08 PR
2012 winner: Mariya Savinova; 4:08 PR
Cain again has run sub 2 and has strength to run 4:04. She would be perfect for the 800m with rounds. Wouldn't win Gold in an open 800m, but a runner in an 800m final after two 800m races with a 4:04 1500m PR is definitely a favorite to win a Gold medal.
If she makes the 1500m team, I don't see her having enough strength to get through the 1500m rounds and still get a medal. She might get a medal in an open 1500m race with a 4:04, but not after semi-finals. A 15:46 5000m doesn't show great strength for a 1500m runner.
And if she made the 5000m team, well does anyone take her seriously at chances to do well at worlds in the 5000m? Especially after a semi-final? Not happening.
Cain/Salazar blew it and put her in the wrong event. I'd love to be proven wrong and have Cain win the 1500m/5000m double in Moscow, but statistically Cain/Salazar made a dumb choice by not having her run the 800m.