I ended up answering my own question when I looked up some rules references to add to my question above. It took me a while and I looked at a couple of books before I found it though, so I figured I'd ask it anyway in case anybody else wondered about this, or in case someone has a different interpretation:
Eventually, I found this in the Rules Compendium on page 212, in a subsection entitled "Distance, Specific Destination, or Both" (emphasis added):
... When a distance is specified, it is a maximum... When a destination is specified it is absolute ... move that target to that destination or not move it at all ...
and, a little further on (emphasis added):
For instance, a character's power might say, "You slide the target up to 5 squares to a square adjacent to you (or "5 squares to a square adjacent to you)," both of which mean the character can move the target up to 5 squares, but only if the move ends in a square adjacent to that character.
So, I would say yes. I can shift to the enemy, attack them, hit, decide to shift only zero squares, and then decide not to slide the enemy at all.
Furthermore, if I had a feat that let me force additional movement, like Battering Shield (assuming my swordmage wanted to use a heavy shield for some reason...), then although I could technically slide the target up to two squares, the slide would have to end in the square I had occupied, which would be adjacent to the enemy's starting position.
Similarly, if I had a feat that let me shift more, like the Mark of Passage, I could shift 2 to the enemy, attack, hit, and then shift up to 2 away. So I could choose to be adjacent to the enemy after moving them, or not.