question 1: there are Iodine loading tests that you can take - i don't know a lot about them, other than it measures the amount of Iodine present in the urine, logic being that the more saturated you are with iodine, the more you will flush out. not sure if there are options for taking that test over in UK. depending how deficient she is, it can take some time to saturate the body.
question 2: no clue on any potential 'issues' with apple cider vinegar & iodine. i doubt there is much to be concerned with there, but hopefully someone else chimes in.