Why does Clemency get interrupted? And why does Sword/Shield Oath reset combo?

[Februs]

Incoming spike damage pld switches to shield oath, damage is mitigated immediate increase in effective health and effective healing.

This is not accurate. Increase in effective health... maybe (though not exactly. It's more like increase in Hp Mitigated. Effective Hp would also include expendable HP. A Pld gets 0 additional eHP from switching Oaths), but even if you consider Shield Oath a increase in effective health it is most certainly not an increase in effective healing.

Shield Oath does nothing to increase a Pld's Hp pool or increase the amount of Hp restored by a Healer. The potency of the healing they receive is not changed by Oaths (unlike Wars). They might take a shallower hit because of the Oath switch, but it is not easier for the Hp to get restored. In that Regard, Wars have an unquestionable advantage. They may have to use an additional skill to take full advantage of that eHp (in an nightmare world in which your healer completely ignores you), but even that is not an issue, because none of a War's Wrath/Abandon moves interrupt their primary combo, nor does switching stances. Switching is seamless.

In all honestly, there really is no excuse (except carelessness or laziness) for a War to switch stances and not have a CD at the ready for immediate use, especially with Infuriate at their disposal.