And yet the mighty dragon will not be on the PCs doorstep coming after them, and if its lair IS easily accessible to them it will have a large easily read "here be the big dragon what will eat'cha if you goes in this here cave" sign. Sure, you might make up the dragon's lair without knowing exactly when and why the PCs will venture there, but you'll darn well know they'll be something like "around level N" and if not you're going to be giving them hints, plot hooks that lead elsewhere, etc before they hit the door.
Not necessarily. After all, why would such a dragon kill a PC that presented no threat? And why would a dragon have a lair that a low-level character could find/walk into? A more likely progression is that characters of radically different power levels will encounter each other, but that the terms of that encounter will be dictated in part by their abilities and it will proceed based on what the two think of each other.
It is just not practical in a D&D game where there is such a wide power curve to do otherwise.
Combat is only likely occur if both parties think they can win and want to defeat the other guy for some reason. And once it starts, if one side is losing badly, they'll very likely be in run/negotiate mode.
That aside though, I do find that many of my encounters, if I had created them under the CR/EL system, would fall outside the levels where there even is a suggested XP award. I don't find it uncommon at all to fight a battle where one side's basic combat numbers are higher than the others by double digits or similarly extreme disparities in special abilities exist. The game still plays fine. World doesn't stop turning.
For low level PCs the vast majority of the things they could run into are instantly lethal.
To be fair, this is one reason why I like vp/wp. The one-hit kill factor is vastly decreased.
However, even that IS the same sort of thing.
Again, not necessarily. While in many cases, you might be right that characters of disparate power are unlikely to fight to the death and a high PC mortality rate bogs the game down, I don't think the game is much fun unless, at least on occasion, the PCs are legitimately up against the odds. I find it quite thrilling as a DM to throw a battle at the party that I truly don't think they can win. For a variety of reasons, those usually turn out well.
To be fair, I occasionally do the reverse as well, though no one ever seems to talk about battles that are too easy.