Stop it!!
Fire that "mentor" aka obsessive, manipulative, quilt police!! No wonder she offered so enthusiastically to "teach" you without pay...an opportunity for her own agenda easily instilled without questions.
My word, how DO we let these people loose on the unsuspecting quilting newbies?!! They should be required to wear signs around their necks.
(Okay, rant over. )
It is fine to add multiple borders one at a time.
It is fine to add multiple borders all at one time after sewing them into a strip set.
It is fine to MITER a strip set of multiple borders. In fact it is a slightly more advanced skill that produces a "graduate school effect" as my old mentor used to say. BUT it is not required, for Pete's sake.
Mitering takes a bit of practice as angles are necessary, butting seams are useful, sewing to a specific point (often marked with a pencil dot) is wise. But it can be learned and you will be delighted with yourself when you've done it!
Jan in VA