in + Abl: answers to “where”
in + Acc: answers to “where to”
So: “Romani ite domum”....
“in + Abl: answers to “where”
in + Acc: answers to “where to””
Actually, your example (which does not use the preposition “in”), does not really illustrate your point, which is nonetheless valid. In your idiomatically correct example, “Romani ite domum”....”, the accusative case implies “ad”, “Romans, go (to) home.
“in + ablative” is usually translated as “in” while “in + accusative” is usually translated as “into”. I am not sure if “caput capitis rectum” is an idiomatic Latin expression (I have never read it). If it is, then I stand corrected. If it isn’t, I guess the correct grammar would depend on exactly what the writer wants to say “He has his head in his a$$” vs. “He just stuck his head into his a$$.”
In any case, Latin is a much more compact language than English, isn’t it?