Better late than never! I like the idea of a new resource in the game, as there are so few ways to constrain in-game economy without over-constraining it..
1- I guess if the power source and elerium blows up, recovering what's left, scattered all over the place, might not be worth it.. was just a thought to try to make it more available than Elerium but I agree it doesn't quite work.
2- Although I love talking about XCom weaponry, I have to admit having little interest in its real life counterpart. It's good to have someone knowledgeable sometimes
For crafts, more weapons is better, although there are only few parameters to tweak, so it is hard to not make them redundant after a while. Cannons are covered pretty well with the alloy ammo, gauss and improved laser but missiles have seen little attention. I only know of my Elerium Missiles, DE ones would be great.
3- I guess you don't like the ap rocket idea. Maybe I'll have to pursue that myself. Otherwise, maybe using DE as an upgrade ammo to gauss weapons? Gauss weapons work by accelerating chunks of heavy something at the enemy. DE would be heavier (better!) If the basic gauss is an equivalent to lasers, maybe gauss + DE ammo could be the equivalent of Elerium focused lasers?
4- I like advanced interceptors too, that's why the XCom Armoury Expanded - Crafts, makes use of all 5 new fighters (Alloy Interceptor, Retaliator, Thunderstorm, Raven and Sentinel) tweaked to answer different needs and offer a smoother progression, up to just under avenger power-level. I'm just not sure DE armor adds enough to justify stretching cannon (suddenly, alien alloy isn't enough, and the UFOs and vanilla new crafts are using subpar material for armor) and balance (if it's tougher, then UFOs become weaker by comparison).
There's always space within the current limits (from conventional damage to plasma/blaster damage) for more manufacturable ammo, since you always have to manufacture some any ways. Hence why I suggest DE ammo for gauss weapons. But weapon type, armor, crafts and tank platforms are different. If there are too many, at some point you don't have time to finish manufacturing one and already discover the next best thing. So you end up skipping some things in between and all that work goes to waste.
If you don't fit the new stuff within the existing limits and add a new "best" weapon type/armor/craft/tank chassis, then the aliens become weaker in endgame unless you upgrade plasma weapons/armors (harsh on the early game) or give them new weapons and UFOs for their endgame (complicated, but not impossible).
Sorry for the long post. I hope it doesn't come across as overly critical. I like the idea, and would be happy to help with the ruleset if I can (leaving for a trip in.. 2 weeks..!)