Re: EC's cannot have non-END powers!!!!
My two cents.
The only change I would make to an EC is to make it function somewhat like multiform, in that you pay a base cost based on half of the most expensive power in the EC, yet still charge the other half of each power. So if you have a 30 AP power with a 40 AP power in the same EC, you pay 20 points base cost, 15 points AP for the weak power, and 20 AP for the strong power. An EC may have some "tipsies" based on unequality, or worse, a future rule change that changes the AP cost of a component of an EC that is currently compliant with rules.
As far as EC's are concerened, I view them as a cohesive unit that is bound together in a unit. Take Spyro for instance. His many breaths are good, but I find it plain silly to require them to all have the same AP cost just because an EC says so. THe player may legitimately wish to have a weaker secondary weapon, such as water breath (which is relatively harmless and doesn't cause damage) versus a lighting bomb (seeker missile).
Themes like this almost require AP differences. The base cost seems to cover the EC-ness of the group of powers, and binds them together.
As far as I'm concerned, an EC is a single power that just happens to have more than one possible effect. Which is why the cost savings and neat packaging are probably offset by the "one for all and all for one" nature of drains affecting the entire EC rather than just a single slot in said EC.
I've never GM'ed in practice, but I find flexibility in my interpretation of "GM prerogative".
As far as END costs, you are allowed to use a power that costs END, even if it has the "No END" advantage. Perhaps native (aka, base) powers that don't cost END are also inappropriate for inclusion into an EC to begin with, advantages and limitatiosn that affect END usage to the contrary not withstanding.