Guard power issues in OP has alas always been an issue. If you encounter problems in PVE I rec you should use the trait and legacy for power restore on parries. Also sweeping cut and stagger are bad power drainers. Avoid using Sweeping Cut (stagger is uber skill). If you encounter such a prob in PVP try running around with 2k ish power and 500+ icpr. Traiting for power restore in the moors is pretty much a waste, creep finesse is high enough to guarantee you won't parry more than 2-3 times in a 1v1 so wasting a trait slot and a legacy for 150 power per fight is kinda worthless. Other than that, your only friends are: Power pot, Take to heart, Thrill of danger.
Note: You can trait for Catch a breath too, for 2 reasons. If you' re traited for Force opening, whenever the skill crits, it gives you a block response meaning you can use catch a breath to heal or get power too (if traited). Same goes for ignore the pain (the skill that takes away 3 wounds), it gives you a block response so you can use it for power too (if traited for CaB).
Courage and honor are the two greatest qualities of mind.
For sword and board, there's the reactive traits(max your legacy, use both traits and make sure your B/P are 20/20ish at level cap). Otherwise you have all the standard options; thrill of danger(use w/2-3 foes present), take to heart and catch a breath(max your cooldown legacy and trait for it obviously). Lastly there are power pots(which use should always have on hand) and cooked food. If these options aren't enough for you then the above posters are right and you need to start using skills when you need to use them and not purely out of habit or compulsion.
For overpower, there's not a lot you can do. I aim for low base power cost increase by equipping the laighathel set from orthanc(-10%) and using the -6% legacy. You can also avoid the raw power trait because it's roughly +800 physical mastery for a base power cost increase of +5% overall. Beyond that .. ICPR, cooked food and power pots are your friend.
You can, but it's not a great return. It's often better to stack raw physical mastery than might and raw crit chance rather than agility--albeit, I do stack agility to bring my hit chance up to par, but it's not a massive amount. .. and in group settings, OP isn't typically the tank unless the instance is under level cap, so your best bet is to place protection by the sword on the MT and hope they parry like a fiend to make it useful.
Myself I don't use the power on reactions trait, only one I use a lot is controlled breathing. That along with pots, take to the heart, that aoe-power return skill (soz forgot name) will do it.
But my advice is to fiddle around. Tank a lot so you get the hang of the class and what each skill does. Doing that you can see what your style is and work around on that. Some grds are very good at single target aggro, less at aoe. They have less power requirements for example. I'm on my best when I have dozens of mobs, using full aoe (yes also spamming sweeping cut :P), so I've been tanking a lot and now have worked out a routine with which I can adjust my aggro around the group... I mean, with a good cappy etc I go more full out so group can dish out more dps and thus pull more. if not, I tend to pull less to maximize the aggro/power consumption. Soz if I don't make sense... hard to put it down since I go with my gut feelings on tanking... I don't have a set routine, I just go with the flow of the group tbh :P