Traditionally saying a person is EASY has meant easy to seduce or sexually promiscuous. But with a good friend who would not misunderstand the context it should be fine.

Yes, but because "I'm easy" has other meanings, you should preface it with your lack of strong preference like: "Either. I'm easy." Used this way, it means you will not argue with either choice, and would easily agree to either choice.

Yes, saying "I'm easy" implies that you are given multiple choices and you're trying to say, "I would be good with any of these choices". I'll take it one step further and say that when you use this phrase, you're generally giving the choice or preference back to the other person. You're basically saying, "I'd be good with any of these choices - you choose".