There should be a "command pet" in the key bindings (PC). I think I have it mapped to 'Y'.
The only time I've used it was to trick friendly NPC's into attacking my bear when Morrowind came out. If you were mounted, they'd turn hostile against the bear, but the bear wouldn't fight back and you wouldn't get a bounty. I haven't tested if it still works.
Yes, if you hold down the 'command pet' key while doing a light attack, no light attack will go off but your pet will go after the target instead. That can indeed help you to get initial aggro onto your pet. If you are stealthed and have some range it seems to work a bit better.
Once engaged, a heavy attack will cause your pet to attack the same target that you are.