There shouldn't be a problem with not adding kalk for 2 weeks. The reading may dip after that time but will gradually return to normal. I think it would be interesting for you to get a reading before you leave and then when you return and see how long it takes to come back.
Most importantly realize if the calcium reading decreases, they will be slowly (over a 2 week time period) and your attempt to adjust should be at least that long. IOW don't try to get it where it was overnight.
As far as going months... again reading will degrade slowly, however I don't agree with the dealer. And I wouldn't worry about the iodine. Actually it would be wise to have someone stop in just to check the tank has water in it.... didn't have a power outage, frozen pump, of clogged overflow. I typically advise to instruct a friend to be able to respond to such an emergency.