Im relatively new but the thing I find is you don't have to play unsafe if you don't want to. I mix in b1, b12, d1, f1, f12b4, 123, and throws in my pressure game. All of that is safe, the key thing is baiting reversals. If you really want to do a 50/50 do it off of the last hit of f12b4. Since the low option is safe, you can do that a couple of times to condition them to block low then hit them with an EX roll the next time.
As far as what to do with b12, if it hits you can run cancel into b12 again to jail. If b1 hits another b1 isnt guaranteed but it'll blow up everything but armor(and if you do b12 it might even break armor depending on what it is).
I've found that playing extremely safe works well because people are so used to Mileena's that play crazy that they just wait for her to hang herself. If you don't do that and play it safe they will just keep blocking your pressure which allows her to do a lot of chip and open them up with throws. Again this is limited experience so take with a grain of salt.