Itching from just the magnesium? or all of those on the list.

Itching can be from dilating blood vessels... when blood flows into poor areas, there can be tingling or itching as the nerve endings wake up.

What is nice about the lotion is that some body areas may be constricted with blood flow so any magnesium in the blood doesn't get into those areas. Using it topically opens up those constricted areas and then healing can happen and toxins can exit. I use about a nickel sized dollop in each arm when I want systemic absorption. It really keeps my blood pressure down.
(inner wrists and inner arms where the skin is thinnest.)

You might be able to use more, but that would be up to you. Some people may absorb alot, and others less so. Those with poor kidney functions have to be careful with magnesium. But I don't think topical use is as dangerous as magnesium based enemas or high oral dosing.