I'm sure I've read several times that magnesium citrate is not well absorbed. I believe what you say about lots of other elements/substances being necessary for magnesium absorbtion. The whole process is obviously quite complicated.
I actually ran out of my magnesium chloride so I started taking magnesium citrate tablets instead. Almost immediately I had my twitching eye come back - which I'd managed to get rid of while taking the magnesium chloride. So I don't think the citrate was half as good.