ProwessProwess is a triggered ability. “Prowess” means “Whenever you cast a noncreature spell, this creature gets +1/+1 until end of turn.”
If a creature has multiple instances of prowess, each triggers separately. (CR 702.108),
deathtouchDeathtouch is a static ability.
A creature with toughness greater than 0 that’s been dealt damage by a source with deathtouch since the last time state-based actions were checked is destroyed as a state-based action. See rule 704.
Any nonzero amount of combat damage assigned to a creature by a source with deathtouch is considered to be lethal damage for the purposes of determining if excess damage is being dealt.
The deathtouch rules function no matter what zone an object with deathtouch deals damage from.
If an object is no longer in the zone it’s expected to be in as an effect causes it to deal damage, its last known information is used to determine whether it had deathtouch.
Multiple instances of deathtouch on the same object are redundant. (CR 702.2)
When you
castTo cast a spell is to take it from the zone it’s in (usually the hand), put it on the stack, and pay its costs, so that it will eventually resolve and have its effect. A player may cast a spell if they have priority. See rule 601, “Casting Spells.”
To cast a card is to cast it as a spell. (CR 701.5) Kallist Rhoka, you may choose a creature or planeswalker on the battlefield. Kallist Rhoka becomes a copy of that creature or planeswalker, and that creature or planeswalker becomes a copy of Kallist Rhoka.