I know the card description says "max of 1," but does the card mean max of 1 per card activation or max of 1 for the whole game?
Neither. Max of 1 on the card at a time. If you use it, then you can later put another there.
3. Reduce damage by 1 for each minion on a skewer command (then discard those minions)
...which seems to imply that multiple minions can be on one card.
It doesn't imply that at all. You can have more than one Skewer in your Command Line.