Great looking wheels! I agree, I especially like them on the heritage! Also, white lettering for the win!
We did a bit of research on these wheels when we were considering them for the GTR. When we spoke with the manufacturer they explained to us why they switched from the magnesium to the magnelium and your explanation is correct. The other thing is that magnesium is an unusual material in that it corrodes from the inside out, so you can't actually see if there is a problem with the metal until (generally speaking) it's too late. The magnesium would fatigue or corrode over time and there was the potential for wheel failure.
Now, with the magnelium construction Dymag is much more confident that these wheels will not have the same issues that they had in the past with their magesium setup. If I recall correctly I believe magnelium is a mixture of magnesium and aluminum.
If anyone is interested in these we are an authorized distributor!