Unruly Krasis's second ability will overwrite any previous effects that set the creature's power and toughness to specific numbers. Effects that otherwise modify its power and toughness will still apply no matter when they took effect. The same is true for +1/+1 counters.

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.

The value of X is determined only once, as Unruly Krasis's second ability resolves. Further changes to Unruly Krasis's power that turn won't cause the target creature's base power and toughness to change.

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