The scribe should complete a statement attached to the answer booklet which states what the candidate did for him/herself. So a candidate can tell the scribe that they will dictate all punctuation, for example. Dictating all spelling is really out of the question. Grammar is of course dictated by the candidate.
Last time I heard how marks for SPAG were allocated, it appeared to differ between the boards. Some asked the marker to form an overall judgement, others deducted a proportion for spelling and, if appropriate, punctuation. I do not remember any board specifying exactly what proportion of marks are allocated to each of the three skills assessed.
Hope that is helpful.