Bluff Works - In Character (IC) Only
So long as Elan and Human are indistinguishable at a glance, Bluff will do what you need. If they're obviously different just by looking, you'll also need Disguise or magic.
The harder part is keeping it a secret Out of Character (OOC). If you don't want the other players to know, then you'll have to take some extra measures.
You can get away without revealing that you have poison immunity for a long time, because most poisons allow a save anyway. So even a normal Human can shrug off really strong poisons, and you can just pass it off IC by saying you have a strong fortitude or are just lucky.
OOC, you're going to have to roll the fortitude save along with everyone else. The DM will have to know that you're never affected and handle it accordingly, no matter what you roll. The really hard part here is that the players will notice pretty quickly that you're making saves even with a 2 for poison, while not making saves for other things with bad rolls.
You can handle that either by having the DM roll all your saves for you (if you're the only one doing it, then it's obvious something is up), or when you roll badly, pretend you're affected by the poison when you're really not. This will require IC Bluff checks as necessary (to act like you're poisoned), as well as OOC acting like your character is affected.
IC, you can pretend to sleep. OOC, you can say you're sleeping when everyone else is, and not many people are going to be awake to question it. If you have watches during the night, take one.
A Ring of Sustenance makes this easier as you only have to sleep a couple hours anyway, but isn't necessary.
These only matter OOC if someone wants to examine your character sheet, because until they do that you can just say you have good saves. There are so many ways to get save bonuses that it's not that unusual.
IC, it should never come up.
Power Points to Prevent Damage
IC, you are lucky and some of them just nicked you.
OOC, this is getting trickier. You'll need an agreement with the DM on how to use these without revealing that you're using them to the other players. Maybe you can just do it without telling the DM you're doing it, and do the tracking yourself. That's how I would handle it in the game I'm DMing right now.
The Big One - Being an Abberation
This is actually the biggest problem. You're not a humanoid. Lots of spells that affect the humanoid type won't work on you at all. If someone casts a spell that never fails like Enlarge Person on you and it suddenly fails, questions will be asked.
You'll want to try to suggest those kinds of spells always be cast on other people "for the good of the party".
Have An Endgame
You're probably not going to be able to keep this up forever. IC, you'll fail bluff checks at some point, and the party will catch on. Maybe they'll actually check you for poison, or try to hit you with a buff that fails due to type, or so on. When it happens, you'll want to have a plan for how to explain to them why you kept it a secret.
OOC, the players may notice at some point that something is up. The less you have to have conversations with the DM in private (or exchanging notes and such), the longer you can go without drawing suspicion. So try to work things out between sessions and act as normally as you can during sessions.
But when it does happen, just point out that you wanted it to be a surprise they resolved In Character first.
I've had people do this type of thing in my game (as Changelings and such) and it always worked pretty well. Elan is a bit harder to fit in than a Changeling, but it should make for a fun time.