First, regarding Horrors, you really can't - because there is no such information. Work with the bad things we've got already, because the Horrors are something that should only be happening 1500 years down the road. Understand that out of game, SR is trying to stay away from ED links, due to potential conflict of intellectual property rights.
Second, in regards to Burnout / Lethe / Dunkelzahn, remember (if you know) or understand (if you don't) that if you're going to use him as that, he is not gonna be 'in the game' for any real period of time - the Dragonheart Trilogy put him firmly in the astral and the metaplanes, wandering around and squashing potential spike points and thus keeping the Horrors from crossing early. Having him show up, suggest something from the Will, and disappear is cool; having him pal around with them is kind of ... breaky?
Anyhow.
Third, in regards to the photos, you need to pick up the adventure booklet called 'Missions' for 2nd Edition, FASA 7325. In it, there is an adventure which discovers the facts about those photos; you could actually run the team through that adventure BEFORE you send them off to Mars, because the WIll talks about finding out the truth about them, not about going to Mars.
[spoiler]As I recall, they're fake. Sorry.[/spoiler]
Fourth, the Illuminati: like in real life, in Shadowrun they don't exist. Or at least, if they
do exist, they exist under some other organization's name, some of which you've mentioned - the Black Lodge, the Ordo Maximus, the Catholic Church, whatever. When it comes down to it, if you want to use them, you're going to have to create them and manage to insert them and their tentacles into the Sixth World all on your own.
Fifth, regarding Toxic/Tainted, no, they're not the same thing. I personally would step away from the Ukraine / Chernobyl / astral cocoon and the Force Zillion toxic spirit - because this is an entity that a) is well above the player's capacities, and which b) is going to draw the attention of Goldensnout and all the rest of the Great clan. Realize that Feuerschwinge (Firewing) was a toxified GREAT Dragon (explore some info on her
here, and there were not one but two other Great Dragons as well as at least one adult dragon that got into it with (or in defending) her; that's just a Class I horrible destructive mess waiting to happen. Instead of creating this, however ...
... use Aztechnology.
No, seriously: use the AZT. One of the provisions of the Big D's will is this:
To the first party to determine what lies behind the door of room 5B78 of the Aztechnology Pyramid in Tenochtitlán and file a report of their findings on Shadowland, I leave 5 million nuyen or medical care for the remainder of their natural life, whichever seems most appropriate.
If you look at the Aztlan Sourcebook, this is the core of what they're talking about, and the very, very strong implication is that what is behind that door is the Smoking Mirror, Tezcatlipoca himself, a Corrupted Dragon (possibly Great). So if you want to throw a nasty, powerful, terrible, very bad, no good, irredeemable villian against them ... set them against Aztechnology and the Corrupted dragon at its heart. If you're interested in doing some reading about how this was handled here in an IC forum, you can start
here in VU93 and go on until
here, at which point it moves to an IC thread
here. While I don't THINK the run's actual action was played out in any thread, it might have been.
That said, understand that dealing with something at that level is going to involve major changes - to the world at the start if you use your corrupted / toxic dragon in Chernobyl, or eventually, should they manage to succeed with purging Aztechnology of that particular issue. Either way, good luck.
