Well I'll keep at it and hope someone on PC parses me.
Can't say I'm the most concerned with GCD uptime when I'm avoiding mechanics on progression as a tank, but lag being a problem is a worry.
Lag is a problem if you aren't pre-pressing your next ability. You want to be hitting the button for it well before it becomes available and the game itself will queue it up as soon as it becomes available to use, which mitigates lag as a problem. Lag is only really a problem when it comes to double weaving.
Your uptime will improve as your familiarity with the fight does. Making sure you are getting your full complement of abilities in under Fight or Flight and also not dropping combos will be a big chunk of DPS.
I think Atonement is a bit bugged at the moment due to the changes to it.
Also you're right that no one uses defensive stats at all. Even tanks only meld Crit/DH. There's never been a modern fight where defensive stats were necessary so no one bothers. Either you can mit it with your defensive cooldowns or you use an invuln if that's not possible.