For expertise, you want to enter in 6.5 as your threshold. Right above the threshold editor is a link to a post with some examples and explanation for each stat:
The difference between those meta gems for defense is vanishingly small – use whichever you prefer. You won’t be able to notice a difference in-game.
Defense is still a decent stat even above the cap where it no longer contributes to getting you crit immune. Some people prefer to go heavier to stamina rather than avoidance based stats… I can play around with the balance a little bit to push it more towards stamina, but it’s not going to have a huge impact on in-game performance one way or the other.
The optimizer is still getting a decent amount of stamina – I tried to set it such that it would go for socket bonuses and get the “free” extra stats rather than stacking stamina to the exclusion of all else after getting crit immune.