Is there a way to either show an already rolled GM result to players, or is there a way to default NPC rolls to show to players? (Specifically, in the Lost Mine of Phandelver, if I click on any of the monster attacks, none of my players are able to see the results.)
Not really. You can change all your npc's to show their rolls however.
I know you can change the default on the character sheet for OGL: This is on the Game Settings page. I don't know if that will change the existing characters.