As for your suggestion its still really only tenuously related to darkness at best. And the underworld is still far more closely related to death then darkness. Especially going by Elements definitions.
A movie has no bearing on this game and shouldn't be mentioned.
HoM&M has no mechanical relevance to Elements. It only had such relevance when it was the decided that the Wonders system be used.
You just said
"Might and Magic had no mechanical relevance until something was decided that gave it mechanical relevance."
How that in anyway combats my point I have no idea.
Using your own words, it translates to this:
"HoM&M is only tenously related to WoE. It has no bearing on this game and should not be mentioned."
By using the Wonders system, HoM&M becomes more related to WoE and gains bearing.
Which means that an idea, unrelated to this game and with no bearing, should be mentioned for it can become related to this game if it ever becomes accepted.
If you still don't understand that, well... just review the posts.