Why घर before कब in this sentence?
Both are fine. Putting कब after घर maybe stresses the question a little more.
I have seen other Hindi guides say that the future tense of जाना in this case would be जाएगी ie without the य - is one wrong? Or are they both fine?
जाएगी is the correct form. But people tend not to bother which spelling they are using because in pronunciation, both sound nearly (not completely) the same.
You are right
य-वाला गलत है| जाएगी - वाला सही है|