The elves in ED were a thorn in the dragons' sides for a couple reasons. For one thing, the immortal elves had a 'falling out' with their progenitors, not wanting to be slaves, and there was still bad blood on either side by ED times. Things weren't helped any when Alamaise (he used an 'e' back then) showed up at the dawn of the 4th World at the site of one of his old lairs, which happened to be the wood where the Elves had taken to making their Lothlorien (named Wyrm Wood) and said that he was the ruler of all these lands, and they should serve him. The elves weren't amused, and he killed some of them, including the current queen. Oh, and then Alachia, one of the more Machiavellian and petty immortal elves, ended up as the Queen of the Elves. No, I'm not making that up. So basically, it boils down to the fact that Alamais was always a dick, and people hated him.
And yes, dragonkin needed a regular infusion of dragon blood (in a Rule 34 sense, in case anyone was unclear) for the line to continue producing dragonkin. However, if you were in the range where you were able to produce dragonkin kids, just being really old wouldn't have much affect on that, as Ehran proved by having a kid in the Sixth World. I suspect that, unless there's substantial biological changes, males would remain viable parents longer than females. If I have to explain further, I'll just send you back to high school health class.
And yes, dragonkin typically have some kind of draconic mutation, such as scales over parts of their body. However, even without magic, depending on where these mutations are, they could be easy to hide. In the Sixth World, they're even easier to disguise through mundane means, to say nothing of illusions.