Why does Shurelia appear in Misha's and Aurica's Cosmosphere's?
It's because Shurelia's will and personality can subconsciously seep in trace amounts into the other Reyvateils' Cosmospheres due to her being the Tower Administrator and residing into the Tower's Binary Field.

That said, she herself can't consciously see the events taking place in the Cosmospheres or interfere in them because her SH Server (the storage device where the Cosmospheres are lodged) is separate from Beta-6D's, which is the one where all other Reyvateils have their minds stored. And since the Beta-6D SH Server was added after her Administrator training was complete, she doesn't have any power to interfere with it either

(Reference: http://artonelico.wikia.com/wiki/ARM_Backup/Ar_Portal_translation/Toukousphere/Issue_46)