Questions regarding PLD skills...

[Phoenicia]

Wanted to add, silence works against magic only. There are fights where it is NEEDED, most recent being Ultros that casts a 300% of its health shield (or something ridiculous like that) and Seiryu adds that cast AoE stoneskin that greatly hampers the DPS check at lowish gear/skill levels.

Low Blow > Shield Bash.

Low Blow is oGCD (as mentioned by Kalise), free, and has a short CD.
Shield Bash is on the GCD, breaks combos and costs a whooping 120 TP. (activates faster but you have to wait for the GCD to even use it.)

For your third question, there is no "best" defensive CD rotation. You tailor your usage to:

a - Nature of the fight. Damage intake over the fight's length.

Is it a pack of trash mobs? These usually have higher damage in take that gets lowered as mobs die off. So bigger CDs first and weaker ones later. I tend to pop Hallowed in one of those.

Is it a boss? The next point is more relevant:

b - Solo tanking vs co-tanking and what their job is.

If solo tanking, bosses that are designed to be with one tank tend to have relatively low damage or highly spaced tank busters. In the first, you want to use shorter CDs like Rampart first so they come back first. In the latter you want to mitigate more via Sentinel.

In co-tanking situation, it helps to know your tank jobs' capabilities.

PLD has very effective buffs but very long cooldowns. Warior can cheese through a lot with its cheat death holmgang (which has the same CD as your Sentinel) while DRK will rotate its relatively shorter CDs more with TBN on top.
You probably want them to start the fights so whatever they use comeback earlier. You can also Cover them to take a buster in their stead without tank swapping, though this option is not always recommended because less Shirks mean less aggro boosts and cover has other uses that can help non tanks handle mechanics where you take the damage instead of them and at a much lower healing requirement. So it is fight dependent.

EDIT: PLD has the unique ability of CD sharing through intervention, in many situations where both tanks tank damage or share a buster to split its damage it might be worth considering a CD share to save a CD on your partner. My static does this in O12s as a means to save a CD for my co-tank to use to mitigate more damage elsewhere.

Hope this helps