Re: how common is it for a 5 y/o to read backwards?
Common. She is young. My son turned 6 at the end of December and is in kinder. A year ago (so he would have been your LO's age) he wasn't even interested in reading books (loved having them read to him!). He just wasn't ready. A year later and he is reading books, sounding out words, writing words, surrounding himself in words.
It probably has to do with her age, she is a very young kinder student. Most kinder students have concepts of print and are reading/writing sight words. What have her teachers suggested to you?