Basically, Convalescence only affects the Cure series, regen, physick, adlo, Benefection series, and aspected bene. Any other abilities are not affected.

And regen is treated the same as Protect or any other buff you have. Everything is calculated at the time of casting and is set based on that time. That's why divine seal+regen persists even after divine seal wears off, and why when you sync to a FATE, your buffs disappear.