If you want to say to go past 535 defense for avoidance you may as well say to go past 540. Going for avoidance dodge is better than defence by a little bit, so ideally you'd use the extra defense on dodge rating instead. Getting uncritable is very nice, but isn't a requirement. You should gem/enchant for it if possible, but just because you're a little short doesn't mean you can't tank a heroic. Don't forget resilience(the pvp shoulder enchant isn't bad, 30 stam 15 resil), but use defense as much as possible due to the avoidance it gives.

Block is good in heroics, but not against raid bosses that hit hard, so for heroics block is fine but if you want to go into raids you'll eventually need to switch it out to avoidance.

For progression, EH has time and time again been the preferred method for gearing. That means gem for stamina after you're uncritable. Of course meet your meta and sacrificing some stamina to get a socket bonus is justifiable. All gems should typically be 24sta(blue), 8def12sta(yellow), or 8dodge12sta(red), but going for extra defense for uncritability if needed is good too.

Strength is your threat stat, and also adds some nice block value, but threat shouldn't be a problem.

4x lhw spam should be able to keep your tank up, but you lose all of your dps when you do that. A resto shaman keeps 3 dps going 100% of the time.

I doubt heroics were meant to be a cakewalk the day you hit 80.