The only thing I'm uncertain off is when armor mitigation kicks in..

Damage Reduction (generally accomplished via armor but given through talents and a slew of other things) is factored in last, the difference of which (whether K/E or I/E) is determined by the kind of damage dealt by the attack.

Defense rating is the chance to dodge ANY attack.

You're wrong. Defense rating only applies to your defense chance, which only applies to melee/ranged (re: white damage) attacks. Your resistance chance (re: defense chance that applies to Force/Tech attacks, also known as "yellow damage") is completely unaffected by any of your stats and can only be increased by powers, talents, or set bonuses, and, even then, is generally increased by only small amounts (VGs and Shads can expect to see 2% resist chance; Guards don't get any).

After defense the attack is either shieldable or non-shieldable.

Just like with Defense chance, the difference in shieldability is based on the type of attack: melee/ranged (re: white damage) attacks can be shielded while Force/Tech (re: yellow damage) attacks cannot. It isn't some arbitrary determination that is hard to acknowledge.

Something to keep in mind for PvP (since NPCs cannot crit) is that the crit chance and shield chance use the same percentage roll to determine whichever happens and crit takes priority over shield. For example, if you have a 50% shield chance and your opponent has a 20% crit chance, they're both standard. As soon as the combined crit + shield chance is greater than 100% however (say, 60% crit chance and 50% shield chance), the crit chance pushes remaining shield chance off (60% crit and 50% shield end up being a 60% crit chance and a 40% shield chance).

TL,DR There's no clear cut way of ever saying that one tank stat is better than the other.

The best advice I can give for tank stats is to ensure that your gearing strategy matches your healers' style of healing. Both mitigation and Endurance stacking gear strategies (as well as a whole spectrum of gearing strategies between the two) have been demonstrated to be effective at clearing all of the content in the game in roughly equal measure. Some healers prefer to constantly keep the tank topped off; these healers operate best with high mitigation stacked tanks. Some healers prefer to minimize overheal by keeping their tanks stay at ~90%; these healers operate best with high Endurance stacked tanks. Some healers, from a psychological perspective, simply prefer to deal with higher hp pools. If you're really interested in optimizing yourself, ask your healer(s) whether they'd like to see you with more hp or more mitigation (if you're up for it, tweak your gear a bit one way or the other and ask them whether it was better before or after).