If a creature somehow loses all of its +1/+1 counters, it can adapt again and get more +1/+1 counters.

Cursed Wombat's last ability grants a triggered ability to itself as well as other permanents you control, even if those permanents don't have power and toughness.

If another permanent would enter the battlefield under your control with one or more +1/+1 counters on it, the ability granted to that permanent by Cursed Wombat will trigger when that permanent enters the battlefield.

If Cursed Wombat leaves the battlefield while one or more triggered abilities generated by the ability it grants are still on the stack, those abilities will still resolve as normal and put additional +1/+1 counters on the appropriate permanents.

You can always activate an ability that will cause a creature to adapt. As that ability resolves, if the creature has a +1/+1 counter on it for any reason, you simply won't put any +1/+1 counters on it.

If Cursed Wombat leaves the battlefield while a spell or ability that would put one or more +1/+1 counters on one or more permanents you control is still on the stack, those permanents won't have the additional ability granted by Cursed Wombat when those counters would be put on those permanents, so that additional ability won't trigger.

If you control multiple Cursed Wombats, each permanent you control will have that many instances of the granted ability. These abilities are not redundant. For example, if you control two Cursed Wombats, each with no +1/+1 counters on them, and you activate the adapt ability of one of those Cursed Wombats, you'll put two +1/+1 counters on that Cursed Wombat, and then each of its granted triggered abilities will trigger. When those abilities resolve, each one will put an additional +1/+1 counter on that Cursed Wombat. Since those abilities trigger only once each turn, those additional counters won't cause those abilities to trigger any more that turn. (Curses!)

If Cursed Wombat leaves the battlefield and returns to the battlefield in the same turn, or if one Cursed Wombat leaves and another Cursed Wombat appears, the ability granted by the old one is different than the ability granted by the new one. This means that a permanent that already got an additional +1/+1 counter from the previous granted triggered ability could get another additional +1/+1 counter from the new granted triggered ability if more +1/+1 counters are put on it during that turn.