Now, to start crossing relics off the list. Dread Guard clicky relics share cooldowns, so you only want one. The Defense click relic gives better mitigation to a Jugg than a Shield/Absorb click, so the Shield/Absorb click is tossed out.
No, they don't. Identical relics share the same CD but different relics (different tier *or* different stat) simply put the unused relic on a 1 minute cooldown. As such, if you have 2 use relics, you can use them both on CD, once every minute without any loss to uptime.
You're also forgetting something *very* important about the DG use relics for tanks: they only have a 20 second duration. You actually get better returns out of the Rakata Defense relic (290 Defense for 30 seconds every 2 minutes = 72.5 Defense on average) than you do out of the DG Defense relic (350 Defense for 20 seconds every 2 minutes = 58.33) because of this. It's one of the reasons why pretty much no one recommends the DG tank use relics any more: the previous tiers are actually *better* (for use relics, the Campaign relic is best for Shield/Abs and the Rakata is best for Defense; the DG relics are both major downgrades).
the math is even more in favor of a Matrix Cube providing all around more mitigation than the Absorb proc
While it's true that, for a Guardian tank, the Absorb Proc is friggin' terrible, saying that the Matrix Cube vacillates between "slightly better" (1.2% less damage taken) in high attack/second scenarios to "slightly worse" (.25% more damage taken) in average scenarios isn't the same as "math is in favor of the Matrix Cube providing more mitigation", especially since you're only factoring in *basic* attacks rather than any of the special M/R attacks that are going to have substantially less real contribution from Defense (so you've ended up inflating the value of Defense by a fair margin). If anything, since the situations you should care about as a tank are those where you're predictably taking a crapton more damage, weighted properly, the Matrix Cube is actually substantially worse. Even so, at worst, it's functionally a break even, and, at that point, it's not because they're both *good* relic choices: it's because they're both *bad* relic choices.
TThat leaves the DG Defense click, War Hero Defense, and Matrix Cube as the three more viable relics for appropriately geared Jugg tanks
As I've pretty well demonstrated, there's only 1 real "viable" relic for Guardian/Jugg tanks and it's the WH/EWH passive PvP relic. The Matrix Cube and the Abs proc relic are both terrible, terrible break evens and the DG use relic is, at *best*, marginally better than either of those. All 3 of them are *worlds* worse than the WH/EWH passive relic though so there's no real option otherwise: if you're not using the PvP relics, you're using painfully subpar ones.