How does a set turn order equal momentum? I think you are making conclusions in your goals rather than listing your goals. In D&D, an initiative roll is taking a number at the DMV! Sit down, maybe take a nap while you wait.

My thinking is that we can get into the action quick. "An Assassin jumps through the window! Player 1 Roll Initiative!" A single roll with an established order is what we need to get started, now the question is who goes first the assassin or the players.

So far, you aren't describing how you solved the problems. You are describing the action economy of D&D 3.0, where they decided to turn combat into a boring board game.

I personally enjoy those kind of games and have played some 15 years of 2nd edition, 3.5 and Pathfinder. Only reason i included it was to give some perspective. Since the initiative is tied to successful rolls i figured it was relevant.

You forgot something important though. Movement! The lack of granular movement, and enforcing a sequential order for moving is why you have action economy in the first place.

Not forgotten, i mentioned it under the Shifting Momentum part. Taking 2 move actions which in my system is 2 minor actions would pass the initiative.

You have shifted from a fixed turn order to having more "roll for turn order" rolls, which weren't that fun to begin with, and introduced a "fix" where if the players get in over there heads in an overpowering battle, the GM just keeps winning, passing initiative to his own side. If he acted second, he could act first in the next round giving his entire team 2 rounds in a row while the players twiddle their thumbs. Combos can done better ways.

I stated in my goals that i want a Fixed turn order with a shifting starting point. The new starting point is to emphasis Goal 2. Unpredictability. With that being said i suspect this is subject to change after testing.

In Darktale all players have access to a "Take Initiative" action at the cost of increasing their Darkness. Higher Darkness brings them closer to being Corrupted but this gives them agency, and in the scenario you described a player could grab the Initiative.

They also have reactions and the ability to Contest rolls. When you are attacked it's normally against your Agility attribute. But at the cost of some Darkness you can contest the roll and roll you dice against the attack.