I believe that Hypertime has been around all along, from the beginning of time, in the DCU, even before the Crisis. I think the pre-Crisis Multiverse existed within a particular Hypertimeline, but not the ONLY Hypertimeline, and that this Hypertimeline is the one that now contains the mainstream DCU. In this Hypertimeline, the Crisis still happened, the infinite earths became one earth, and history was retroactively altered to reflect this. There are other Hypertimelines where this did not occur, where the Multiverse lives on (and I think we're seeing images from these Hypertimelines in Kingdom #2, not technically images from the "main" pre-Crisis Multiverse). There are other Hypertimelines where there NEVER WAS A MULTIVERSE, and still others that represent Elseworlds and Tangent stories.
Just because we can access pre-Crisis-like worlds through Hypertime does not negate the effects of Crisis within the primary DCU Hypertimeline.
Are continuity errors avoidable? No. Even in a case where ONE writer is responsible for crafting a series' stories, mistakes are bound to happen because the writer is human, and human beings tend to make lots of mistakes. Image Comics' Spawn has (until a few issues ago) been primarily written by one man, and yet I could point out at least 10 MAJOR continuity errors within 70-odd issues. For a company like DC Comics, which has a 60+ year history, with legions of writers and artists and editors crafting the tapestry of its universe, adherence to a strict continuity is nigh impossible.
The company no doubt thought it had fixed the problem with the Crisis, but in hindsight this was pretty naive. Shortly after the Crisis, every editor and his mother took a notion to revamp the characters in their charge. Beginning with The Man of Steel, which was a brilliant and possibly necessary reboot of Superman, retconning came into vogue at DC.
The biggest example of this, and one brought up every time this sort of discussion takes place, is Hawkman. By the time Hawkman was revamped, he had already appeared in the Superman books, among others, as his "old" self. So there were, apparently, two versions of Hawkman running around in a universe where different versions of characters just didn't exist.
The Hawkman problem began to snowball, and other continuity errors popped up all over the place, and so in steps Dan Jurgens with Zero Hour to fix everything. Except Zero Hour didn't work because more mistakes happened, and... Well, you get the point.
With Hypertime, continuity errors can be explained away by saying that a divergent timeline, containing slightly different versions of certain characters, made its way back to the "main" one, affecting small ripples of continuity distortions in the DCU. Mark Waid has commented that, for the most part, the only people who will notice these distortions are the fans, because presumably if a divergent timeline causes a different Hawkman to exist, then every DCU character is different too, as a result. Superman, for example, would be changed so that he would recognize this "new" Hawkman as the one who'd been there all along.
That is an example of the good that Hypertime can do.
And if it is, then it's up to us as fans to let DC know we don't like it. They do listen, regardless of what some may think.
...and I don't see how that could be a bad thing.