The advantage to shield bash is it has no cooldown. I don't tend to use it often, but you could for example cycle through every add in a pull and stun them one at a time. I would probably go with low blow if you're just trying to say, quickly stun a trash mob to interrupt an aoe. Spamming shield bash on different enemies probably won't really do much good considering the lost dps as you cycle through them.

I don't think I've ever actually used interject to silence. I have it on a bar but not hot keyed and never really use it, maybe there are situations where I should.

As far as defensive rotations go, for me it kind of depends on the fight. I generally consider how long is the fight. For exmaple, if I use hallowed ground on the first buster, I may get a second HG out of the fight, but if I wait, I may not get to use it a second time before the cd is up. Same to a lesser degree with sentinel which has a shorter, but still substantial cooldown. I just try to juggle things so that I always have something up for every trash pull or tankbuster, and tank busters vary a lot by boss in terms of their frequency.