3. It seems pretty good, but I don't think I would personally take one over a tesseract vault. But I can't really comment as I have not tried it out in game, suffice to say it is one tough cookie though!
Yeah, if I could pull it off, I absolutely would. What I'm going for, actually, is just throwing in the allied LoW. With the Decurion, you can actually get away with a single Obelisk in ANY army... it's a legit formation to add one Obelisk and 0-2 Monoliths. Hence wondering about its effect on other factions around it.
My Thousand Sons may use the Obelisk as a count as for a Silver Tower.
The cheapest I could run Tesseract is about 700 points, because I would need a bare bones Lord and a squad of Immortals.