From the Lexicanum:
"The Gellar field was invented along with the warp drive, allowing warp-capable ships and their occupants to actually survive the hostile environment of the warp. It protects the ship and its occupants from the hostility of the warp itself as well as from the predation of warp entities such as Daemons.
The Gellar field device emits a field called a Gellar field, essentially creating a bubble of real space around the ship. The weakening, failure, and collapse of a Gellar Field while the ship is traveling through the warp would be completely disastrous. Warp entities would tear the ship apart to reach and consume the souls of the crew."
Now, I agree that the explanation seems a tad ambiguious but when you add: "The most powerful gellar field in existence", I do not believe Ingethel can penetrate it. I also agree that the daemon telling them not to touch anything served 2 purposes:
1) Make the illusion more "real"
2) Prevent the marines from looking too closely as illusions may not stand up to minute inspection
If Argal Tal and his fellow marines remained loyal, what would probably happen is that the entire crew would be killed. Lorgar would get a message saying, "They were unworthy and not ready for the truth. Send in the next batch."
All of the Chaos Gods were focused on that moment in time, one of the only times that has ever happened. The Chaos gods combined are certainly more powerful than the Emperor. I believe almost anything was attainable by the Chaos gods during those few moments.
All agree that the 4 chaos gods reign supreme in their specific instance of the warp but are essentially powerless in real space because they cannot enter it (Why necrons why to separate real space and the warp for good). In fact, the more powerful a warp entity, the more difficult it becomes for it to manifest in our reality. Which is why the average human host wears out at an alarming clip when possessed by powerful warp entities.
Otherwise, they would have snuffed the candle that is the big E a long time ago. They were powerless, hence the need to corrupt real space entities like the space marines. They are not as powerful as they lead you to believe - at least in our reality.
I infer from the fluff that the Primarchs were essentially perfect and resistant to the predations of chaos. That was why chaos stole them away into the warp so that they become open to corruption. Even so, only 1/2 the group turned.
Lorgar in particular, was bathed in the warp for the longest time. This is so that he will serve as the catalyst for the
HH.
His corruption was not easy as can be seen in his fight with Corax. He wanted to die because he could not reconcile what he had to do to bring about "enlightenment".