I'm 100% sure that Mami and Sayaka will be on board with taking Rionna out. We'd have to actively lie to Sayaka to keep her from going after Rionna with her bare hands. There's no way in the world this is going to be a moral or ethical debate.
Don't take sides, be impartial
This can only serve to make Sayaka and Mami angry at us. Fuck, it'll probably make
Homura mad at us. I don't understand why you think it's a good idea. Maybe you should take it out.
Does anyone have any plan where Sabrina rolls to intimidate?
Not actually do anything but just go all in Godfather on... Irkalla?
She'd punch us in the face and then try to take our gem. We've already seen how she responds to pressure - she escalates.
So the question is: Is it better to attack Rionna now, or to try to get into a timestop and attack her afterward?
The major argument for "later" is so that we can maximize our chances of getting into timestop. Once we get into timestop we have as much time as we want to gather allies, ready ourselves, and line up our opening shots.
I don't find this argument to be particularly compelling. If we let Rionna out of this sphere she will
instantly bring her defenses to full power. What else could she possibly do? She just attacked us. And don't give me any shit about it being "just" mind control.
She attacked us. Flat-out. Unnatural Mental Influence with Hostile Intent. Fucking done. There's no way she's not going to be expecting a fight.
Furthermore, consider that Homura has set herself up on overwatch, and Mami is 100% ready to go. Rionna may be ready, but she apparently needs to
command her spirits, which means that they are in exactly the same posture they were in when we entered the bubble. And when we entered the bubble, her shades were standing down far enough for social-interaction purposes. Consider also that her shades aren't range-limited - once she can give orders to them, she will almost
certainly have one attach itself to us where it'll necessarily come into timestop with us. That's something we really want to avoid.
I think that our best chance goes something like this: Alpha-strike her now. Tell Mami to GTFO. There is
no way that Rionna will be able to counter our alpha-strike well enough to threaten Mami. If Rionna is that badass then we have no chance of fighting her. Clearly we can reject that hypothesis. Contain Rionna inside the privacy sphere. Exit the privacy sphere ourselves and open the distance. Wait for Homura to either get us into timestop (if she can do that without Rionna's shades being pulled in too) or arrive with anti-magic support. That's
more than enough to go into the vote for opening combat.