I'm not 100% sure about this, but if you end a combo with 6c and set up multiple steins, or you're just in the situation where you have multiple steins set up near your opponent, you can PROBABLY 236d, and if he lasers fire enough times you could theoretically have a lot of mixups that are supported by your steins with 236d. I was thinking (though once again, just theory) that you might be able to use an overhead mixup here with 6b as your opponent is forced to block 236d, then if they lasers hit them AFTER 6b is finished (which probably depends on how many steins you have set up) you could then go into 2b>3c>whatever your combo of choice is here. This is just an example out of the large possibility for variety that steins give you, and is just sort of a theoretical example. Not sure if it works, but its the concept I'm talking about here. You also obviously have grab mixups but thats only so usable.
To be completely honest, Mu seems like the character who should be locking her opponent down with proper use of her normals+stein support and thus, be able to punish her opponent's attempts at escaping lockdown. She doesn't exactly seem like the sort of character who has that great of a mixup ability.
I'd like to see her doing things like normals>throw out a stein>more normals>etc and mix up WHEN in her strings she actually throws out a stein and therefore keep her opponent guessing about when they can escape her lockdown. It seems to me like this sort of thing has potential, especially if the corner, IF spaced well since Mu can outpoke her opponents with her heavy normals. When I get my hands on Mu I'd like to experiment with this concept, as well as keeping my opponent in blockstun until a stein I've set up fires its laser, as I throw out another stein AS they are forced to block the laser. I'm sure 214d can be used to continue lockdown as well. I'd love this in the corner.
Also, steins can probably set up for some pretty good tick throws, though I'm not really a big fan of using throws very much.
Sorry for the wall of text lol.