I'm gonna go with the logical approach first.

The eldar want the small skiff, they have seen it on sensors leave the hulk and land on a small imperial vessel.
They haven't actually seen who is piloting it, unless they had a spy on the hulk. Because as we all know, the skiff has hexagrammic wards to blot out ALL psychic powers.
This includes eldar far sensing and other bullshit.
The logical thing for eldar would be to send in a warship, make sure the sloop doesn't get away, WITHOUT FIRING, there's frikkin soul stones on that thing after all.
Then with extreme prejudice get the skiff and leg it.

Since Necroes is right, eldar don't like deamons. But won't risk it over it. The Mon'keigh can have fun with that themselves.

If anyone is getting eldar strike teams after them, it's Zuriel.
Whom as we all remember, HAVE STOLEN TWO SPIRIT STONES.
So the logical thing is if the eldar sense deamons on the sloop, they have a warlock close by (since farseers are rare, there might be one in the fleet) this warlock senses deamons, and thus will also probably sense the spirit stones.
That warlock wouldn't give a flying fuck about the deamons, except considering blowing up the entire ship using his warship. But the wayward spirit stones though..... yeah.....

That's the story approach.
Now for the way i feel.
[@Necroes] I'm gonna be honest, and you already know some of this. You are disrupting this RP.
Not by godmodding actually, as in being more powerful than reasonable. No, by constantly pushing the borders of fluff.

You create the ork, that's fine. A mutation making the spores stop and altering the psyche is very probable, it's the warp after all. It likes mucking with people's heads.
But then later, you claim you can navigate ships in the warp, since you are an ork psyker.
However, being severed from the ork gestalt field, you wouldn't be able to navigate like orks. Since they rely heavily on that to do it.

And the thing with which ship we would take.
You find the eldar strike ship. Which is cool, i don't remember that ship and it actually suited our needs... sort of.
But then this idea of enslaving the crew and forming chaos spawn murder servitors out of them using wraithbone since one can reshape warp matter?
As i said, not just anyone can do that. And rarely does it occur in fluff that characters reshape warp / psychic resonating materials.
I mean, how often does a character reshape deamon swords? Unless A, the sword wants to do it. B, it's a stated ability of the sword which the character is attuned to.
Again, i'm all for messing with eldar. Silly xeno, but wanting to and being able to is very different.

Then it's the event in the skiff.
You bring charged warp essence artifacts into a null zone. Which is fine. Chaos artifacts don't cease to exist in such areas, as proven by fluff with several deamon artifacts.
But then you channel said power within the field to kill the ogryn.
Are you the most skilled chaos sorcerer of all time?

It's the constant "Well, actually!" which is getting on my fluff correctness nerve. It's going crazy right now TBH. So much i myself is even a bit sour over the whole thing.
Constantly finding snippets of weird wiki info to support that your ork can do just this. But doesn't count as an ork and thus isn't affected by this thing, except for when it is useful to, then he's clearly an ork. And other similar events.
Plus the seemingly random acts of kindness and aggression towards Xepherial. I mean, why? Make up your mind.

However, i prefer fluff to take precedence.
We need a logical way off killing off the ork if that is the way people decide on. I myself is actually also neutral in the question.

[@Jbcool] Solutions on how to kill Urgrugg in a fluff correct way, without deus ex GM.
[i]Naval armsmen finally arrive to investigate the prison escape. About a dozen or so. They turn a corner and immediately fire. Urgrugg is taken by surprise and dies.

The ship's astropath starts to freak out, screaming about some sort of abomination of sorts in prison block 4. The captain wanting to shut up the useful psycho jettison the prison block hoping the astropath will shut up then so he can focus on the battle. Urgrugg being this problem, is flung too far from the ship and suffocates before being able to teleport back.[/i]

[i]The deamons bound to his contract finally reach an agreement on how to divide the soul.
Either by splitting it, or by having the one who he sold it too first get it, or by sword to face. In either case, when Urgrugg tries to summon his followers, alas, is greeted with a leering face and is dragged into the warp. Showing that making deals with deamons, is always a folly in the end.[/i]

No need for deus ex GM. There are fluff solutions.

As for the ork staying. Solutions.
[i]Urgrugg senses that the deamons are up to something, and will want to ensure his hold over them and thus withholds using them. Until a proper strengthening ritual can be done. Or the situation is more to his favor.[/i]

[i]Being completely alone on a ship, he realizes that perhaps making some friends is a good idea, after all, that is what a human would do.[/i]

[i]Suddenly, the deamons leave, and with that the powers they bestowed.
Urgrugg feels groggy, and then realizes, he's a weirdboy again.[/i]

I don't like killing off characters.
Especially by means that would in actuality be a raid to kill me, seeing how the eldar will always care about spirit stones first.