It's actually not hard to farm diamonds by watching videos. I do this while farming in Diablo 3.
Not sure if there's a limit to how many times you can watch at once, but I do get a 'no videos are currently available' message after a certain number of times watched. It does refresh, though. I think.
I did spend my first 200 gems on hero summons. Got a few decent ones that I haven't hit a wall in progressing through the campaign just yet. Up to 100+ again today. Next 200 will go straight to more heroes.
Not sure about experience distribution. I could check later. Been farming lower areas for a Sirius, as I have all of his gear and no hero to put them on.