Despite its popularization, iodine has no proven benefits to any of the animals we keep. In fact, in crustaceans (which it is often recommended for), every effort is made to sequester and purge iodine in the exoskeleton during molting. It is absolutely not necessary for the molting process. Stick to calcium, alkalinity, or if it needs to be raised, magnesium for better conditions. Magnesium is actually important for calcium and alkalinity solubility, as well as coralline algae, which uses it quite substantially. Other supplements have dubious uses, as well, including strontium, molybdenum, etc. None of these have any positive evidence as being beneficial to add in any way. Strontium, particularly, is already present in salt mixes inconcentrations several times greater than that of seawater. I haven't added any of the above supplements (except for calcium, alkalinity, and magnesium) in many years and have had zero issues with snails, crustaceans, etc.
On a separate note, the snail problem could stem from several issues. They are often in bad condition by the time they make it to your tank. There could also be a lack of the appropriate food. Most prefer film algae and a lack of it (or too high of a number of snails) will cause them to starve. Predation is another potential problem, which includes hermit crabs, other snails, etc.
Edit: These may be good articles for you to look at:
http://reefkeeping.com/issues/2006-06/rhf/index.php ;
http://web.archive.org/web/20021127040526/http://www.advancedaquarist.com/issues/nov2002/chem.htm
The amount of drop in the other parameter should be undetectable. If you are able to measure a drop, then you are adding too much at one time and/or too quickly. The drop in the other parameter means you are precipitating both out of solution. It is impossible to not precipitate some, but the amount that happens should be absolutely tiny as to be undetectable. So tiny in fact that it should be able to redissolve or be neutralized very quickly. It doesn't matter which one you add first, but you need to at least add the alkalinity portion (I recommend adding both slowly) very slowly (if you are using a 2 part buffer), since carbonate is the less soluble of the two ions. Dosing both at the same time, however, can't be done, as it will result in instant precipitation and you end up wasting an entire dose and potentially lowering both of your parameters as the precipitate acts as a seed crystal to drop more out of solution.