Each chosen mode is performed in the order printed. This means, for example, that you can’t create a Snake token and then put a +1/+1 counter on it.
2019-11-12 • wotc
If you choose the first mode at least once, Pick Your Poison will have at least one target. If those targets all become illegal before Pick Your Poison would resolve, it won’t resolve and none of its effects will happen.
2019-11-12 • wotc