That's assuming he's not clipping his own DoT. Depending on his gear, the current skill speed is enough to allow a Pld to clip their DoT at ~6 seconds, barring no interruptions to the rotation. It's still a case by case issue, but it's another nit-picky thing to keep an eye on in regards to looking out for dps loss. Regardless, it sounds to me like the main problem here is more with the party synergy rather than his rotation.
Firstly, healers who are letting his Hp get "distressingly low" while rotating RoH on top of 100% up-time on Shield Oath rings a few warning bells to me, particularly the RoH part. It's hard to judge without seeing one of the fights first hand, but I have to ask why his Hp is dropping so low. There are only three possible reasons that I can think of:
(1)The tank is not rotating their CD's well enough.
(2)The healers are not anticipating the mechanics.
(3)The healers are too busy baby-sitting the dps.
If the problem is 1, then he just needs to get more practice in. There's only so many ways to fine-tune a tank CD rotation for boss mechs, so eventually he'll get it down. Once he does, he'll be more comfortable swapping stances and his healers might trust him more to do it. At the very least, he shouldn't have to rotate RoH just to keep himself alive.
If the problem 2 or 3, however, then the problem isn't with the tank. It's with the party. This is what sounds to be the main issue based on the OP's description, because, again, he should not need to be rotating RoH consistently just to stay alive. Enmity obviously isn't a problem, because they have a Drk for an OT and their dps is sub-optimal. That leaves either dps who are standing in WAY too much stupid and not scoring enough dps, or healers who are not comfortable with the mechanics of the fight yet and are letting the dmg get too far out of hand. Both of these possibilities are FAR greater dps reductions than anything the Pld can compensate for alone, so there's really not much he can do unless they fix some of the more fundamental issues facing their group. Quite frankly, it sounds like they need to step up a bit. Either that, or they need to step down and be replaced by more competent players.