No, they haven't. Only one has. The other has expressed confidence that they will win. but candidates do that all the time before votes are counted, or even cast, its not the same thing as claiming victory.

> This could have been prevented by simply not changing things at the last minute.

There's almost no evidence that that is the case.

> Instead, we've set the stage for an extremely contested, ugly election.

What set the stage from that is the fact that its extremely close in lots of swing states. Short of abolishing the electoral college or disregarding the will of voters (depending on whether you prefer the more or less democratic approach, compared to the status quo), I'm not sure what you think can be done about that.