Yeah, but 21~roll doesn't have a gap IIRC, so they can just OS a reversal MB flipkick and it will only come out if Mileena does 21 or 21U4.
And yeah, I agree.
What's this defensive OS you're talking of? Because I was going to say that I found out that 21xx low sai/roll is a way to counter MB flip kick, reasoning that these moves come out so fast that by the time your opponent does the input for flip kick they cannot press block in time. Yes these are gambles because of how punishable low sai and roll are but at least there's an option to help keep Cassie players somewhat honest.
Also d3 really is legitimate footsie tool in neutral. Very difficult to punish because of its fast recovery and can open Cassie to whiff punishment because b1 can miss.
Also here's my matchup a notes against Sub.
D2 punishes clone protected non spaced overheads in the corner. Spaced, clone protected overheads can be rolled during its startup. Anytime Sub Zero moves in to either b33 or throw he can be rolled. Be mindful of the fact that they're looking for you to hang yourself, so these are hard reads. Armor works but in some situations lose its armor to a hit and gets caught by the clone.
B33xx clone must be respected while she can punish a point blank b33, if Sub clones, Mileena has no options to stop him. If he ends b33 with d4, the sweep, this is -12 and can always be punished no matter the spacing.
Outside of f4 and b33 Mileena can armor through all of Subs clone canceled strings. However while the issue of breathing hurtboxes making ex not come out consistently during certain strings is real, it does not exist with f42. Be mindful of this.
Strings into ex ice ball can be armored through. A common tactic is for Sub to b33 and use ex ice ball to make it safe, however they can OS this.
Another common tactic is for Sub players to pop an ice clone then throw projectiles from behind it. You don't have to respect this, look for his iceball and punish with telekick. It's easily reactable.
General rule of this matchup, play slow and safe. Subs game plan revolves around playing lame and frustrating you into making mistakes.