Could I target an opponent's creature, then use all creatures from both sides to block that creature, so that Bam! I get rid of something nasty? I think this because the wording on Dangerous says "ALL creatures..."
No. Dangerous's effect doesn't allow a creature to block creatures it couldn't otherwise block, and one can't block one's own creatures. It merely affect the choice of whether a creature blocks or not, and the choice of what it blocks when it does.
Yes, but it wouldn't be useful to do so unless it's something like Illusionary Servant.
The effect would be useless when your target an opponent's creature (except in very obscure situations). For starters, since Dangerous is a sorcery, the opponent wouldn't be able to attack with the creature before the effect ends.