I would not move back (unless you want to). You are not causing your mother's depression. My mother is similar to yours. We were always very close, she really sacrificed to raise us, didn't make her own friends because she had us to use as friends when we got older, kept us attached to her by bribes, etc. This kind of 'too nice' attachment is really not healthy and not normal.

When my mother went through menopause, she became even more difficult to deal with due to the moodiness and random lashing out. There's an end to this, but it may take a few years. I think what you are doing is a good choice.