Mana troops do speed up defense hero’s firing, but in ways that are much more complicated than on offense. Defense gains mana both over time and by being hit with tiles (and by Alberich’s special). There are thresholds for faster firing if the hero isn’t hit (and these are not the same thresholds as on offense). But as soon as you start adding in tile hits and the mana gained, there are not clear thresholds for mana troops—strong tiles give more mana than others, and mana/tile declines during a cascade. See New mana per turn calculation on defense? We investigate
Crit troops add HP; mana troops boost healing. If your healer uses Heal-over-Time (HoT), which are expressed in absolute HP, mana troops are definitely better for healing… If your healer is the more standard sort that heals a % of max HP, crit troops also have an implied bonus for the healing (bigger max HP).
Re Delilah and other minion casters, minions’ HP is always a % of the caster’s max HP. Consequently i always lean towards crit troops on minion-casters to get fatter minions.