Mmm. You need to be careful with conditions like this. Remember that people can always respond, and so whether Explorer is true on resolution may not be the same as whether Explorer was true on announcement. That's fine in cases like Crushing Avalanche or Needle Grass, but ones like this and Twisting Branches have the problem that the number or kind of targets is different in the two cases, but all targets have to be chosen when the spell is cast.

Suppose I cast this with Explorer true, choosing two or three targets; but in response you and another opponent each Boomerang one of my lands. So when this resolves, which target creature is unblockable? The other way round could be even more confusing.

Well, as my knowledge of rules goes. If you still only chose one creature as the target, it would go off fine. If you chose multiple targets and then lost Explorer, one or more targets would become untargetable and the spell would be countered.