If the permanent is already untapped, you can’t activate its ability. That’s because you can’t pay the “Untap this permanent” cost.

If either one of the target creatures becomes an illegal target (because it left the battlefield or for any other reason), the counter doesn’t move. If both targets become illegal, the ability doesn’t resolve.

If a creature with an ability hasn’t been under your control since your most recent turn began, you can’t activate that ability, unless the creature has haste.

This effect may result in a useless counter being placed on a creature. For example, if an age counter is moved from a creature with cumulative upkeep to a creature without cumulative upkeep, it will have no effect on the new creature.

Leech Bonder’s activated ability can move any kind of counter, not just a -1/-1 counter. It can target any two creatures, whether they have counters on them or not.

When you activate an ability, you untap the creature with that ability as a cost. The untap can’t be responded to. (The actual ability can be responded to, of course.)