considering that Misa literally tells Yoh that "it's fine to express your feelings in any form. Even if you're unable to put them into writing... They're clear enough to me." Gotta say this really makes me believe that Misa on some level knows Yoh is in love with her.
Nah, if she knew, she would not tell her constantly how much she loooves Ren sempai. Even if she pretended to not know, I think she would be a bit more considerate towards Yoh. Nah, Misa-s advice is basically aimed at herself, she is the one, who wants to express her feelings (towards Ren), but can't. She just gives the same advice to Yoh, that she'd want to hear.