Magician %/s (KMS FLY 2 & GMS Dawnveil)

It appears that Frozen Orb is beneficial to damage by the smallest of margins (100%/s). Are you actually not able to stack full freezing effects without it?

Yes and more no. All attacks that aren't Frozen Orb or Chilling Step require multiple attacks to stack the effect i.e. five Cold Beams, Ice Strikes, Glacier Chain, etc. to fully stack so that's out of the picture.

Chilling Step on activation (60%) will leave an ice trap that does cause the enemy on it to accumulate the Freezing effect via tick damage, I am not sure how fast it accumulates, DoT rules would dictate that it processes each second, but it certainly seems faster than that, I will look into it. However, this requires teleporting to the the other side of the monster/boss and hoping the effect takes place, then turning around and continuing your assault or teleporting hoping for another go, not necessarily ideal.

Then there is the problem of bosses with "floating" hitboxes, Gollux in particular comes to mind, though there are others like HT, etc.; there, Frozen Orb is the only choice.

If they made Elquines stack the effect, it would be soo much better and it would make sense, well maybe not so much now that it is made of blue fire.

Lastly, Thunderstorm's summon animation feels so cripplingly slow, and literally crippling in certain boss situations, how pronounced is its effect?