They do their job in an amateur mentality. (They are not proficient) Incorrect, but it does work if you replace 'in' with 'with', "They do their job with an amateur mentality." Alternatively, you could say, "They do their job amateurishly." I don't want to be disturbed during my lunch time. Correct, but more common is, "I don't want to be disturbed during my lunch break." I don't want to be disturbed in my own private time. Correct. You can drop 'private' and say, "I don't want to be disturbed in my own time."

They do their job in an amateur mentality. (They are not proficient) They do their job with an amateur mentality. Or "They do their job amateurishly." I don't want to be disturbed during my lunch time. This is correct. I don't want to be disturbed in my own private time. "I don't want to be disturbed during my personal time."