1. There were none to be had. Or none that he knew of. Or they were sleeping somewhere.
Dragons usually sleep for a while. Or they had traveled too far away. Or they went back their homeland, the Last Desert, which is why the
hobbit legend about Wereworms over there exists.
2. This is a subject that I am not entirely clear on.
Sauron is said to have some kind of creepy and magical control over things popularly seen as
evil, but I'm not sure how it works. Sure, the
Nazgul have lost all willpower and seem to be robots, but the
Orcs seem to have free will. So, even though his control is usually written of in a way that makes it seem magical (and creepy), maybe with the Orcs, it just means that they're used to doing what they're told and aren't practiced at rebellion. Dragons are different. Very intelligent and independent. If he tried to use some kind of creepy telepathy on them, they'd resist, they might win, or he'd just have to keep his concentration up at all times, which would be impractical. He'd employ the alliance sort of thing, as was mentioned.
3. Yes, usually. Not always
Dwarves, though. It's just that Dwarves are a very reliable source of such things. And so much fun to play with, since they don't die as easily as the
other races! After Mel was hauled off, the dragons ran all over the place. Lots ended up near stable Dwarvish communities. Lots were killed off. Some were also chained and let loose in
Orome's forest in
Valinor, where he hunts them. Others are in hiding.
4. I don't remember reading that, but maybe you did. Creepy and magical control doesn't seem likely for everything that was under Mel's control (which wasn't always creepy and magical, either). But he was the second in command, so plenty of Mel's minions would have listened to his summons.
5. No problem. Easily understandable.