1) We still don’t really know who or what Beate’s “friend” is. She may simply be Beate’s evil or self-destructive side, or she may be a second personality, or a hallucination, or a genuine supernatural being. She appeared at various points during the flashback. I suspect we’ll learn more by the time this story is finished. Or maybe John wants to keep it ambiguous.

2) The flashback took place three years ago, so Beate did have at least a couple of years to work on this. She was already working on being a detective three years ago, so she may well have developed some impersonation skills as part of her self-training. We don’t actually know how heavy her accent was- plenty of continental Europeans speak very good English. Even so, there have been occasional slips in her dialog that seem to imply a bit of a subtle accent, at least. It’s clear that making her voice sound like Lottie’s isn’t easy for her- that’s why she has the weird squiggly speech balloons when she’s trying to sound like Lottie, and even then, even Dean noticed that there was something strange about her voice.