Unix timestamps ARE NOT guaranteed to be monotonically increasing. They just tend to do that.
No, but what they represent is. I could have been clearer in saying that the epoch is as good an underlying representation as we're likely to get, and it isn't that bad. That makes the unix timestamp as based on the epoch a pretty good thing, in my book.
As you've pointed out, the problem comes from peoples misunderstanding that time durations are not easily discernible without looking up many, many exceptions in many cases. The best we've got is a simple absolute time, and very specific rules for determining what the human recognizable interpretation of that time was at a specific locale at that time.
The solution to this problem is simple; Change the way everybody reads and thinks about time. It's also, IMHO, impossible because simple ain't easy.
1: http://en.wikipedia.org/wiki/Leap_second