on the topic of the good doctor being German - I think he doesn't have a German accent, per se.
- he has a knack for languages. I feel like he knows French and Latin at the very least, possibly others? he was University Educated.
- plus also he's been living in London for some decades
- he studies the Forbidden Language That Sets You On Fire, and that includes speaking it - so his ability to make precise sounds with the right emphases is very good.
I feel like if there's anything unusual about his speech, it actually comes from studying said Forbidden Flaming Language, not from having German as his native language. it's the kind of thing that just worms into your brain. there's no easily-comprehensible pattern to how the sound of it works, but it bleeds into his English regardless - the way he emphasizes certain syllables or alters certain sounds, but only in certain words in certain contexts. it's very hard to put a finger on and the overall effect is "perfectly understandable but uncanny" and people write it off as a product of him being German and mad.
(yes, if you get him to imbibe enough alcohol that his brain calms down, he speaks perfect (Victorian London) English.)