1) Not needed. But, could be nice. Whe can make the same effect with Umbral Soul, but dont make any dmg with it. If whe want to hold ice phase longer.

2) Xeno giving runes dont make sense. It doesnt use fire and with that, is it not fitting to fill it from the lore.
Instant, should all fire spells be able to fill it. (Ok, its not to hard fill it with help if paradox/fire3).
Given more option to fill it, is at last nice quality of life.

3) it takes time to fill all 3 slots, that is correct. But having 3 stacks is nice, because i even overfill them.
Maybe, at a new spell, that use 2 or all 3 of them, but has a longer CD, to not come into conflict with Xeno.