Don't be so quick to write off a Garlean legatus as being a wimpish fight though - Nael van Darnus was after all the final boss of version 1.0 andheshe ended up channelling Bahamut's power throughhimselfherself. Gaius though was sadly underwhelming as an opponent, which greatly disappointed me as I was long looking forward to a one on one fight with him after what he did to my Path Companion in 1.0, and he folded so easily. To say nothing of Regula.
Remember, legatee get to their positions by might not right, so don't discount the possibility of Zenos being 4.0's final boss after all, especially as he's the ruler of not one but two Garlean vassal states (although I am likely to agree it's probably going to be something that comes completely out of left field - this is Final Fantasy after all. ).
I assumed that his foreknowledge of Omega came about because of Roaille, as her name kept being mentioned throughout 3.4 and 3.5's story - given that Omega was being mentioned way back in ARR's story, and that the whole reason Cartenau was being fought over was due to Omega being there (which Teledji wanted for himself), is pretty conclusive it was Zenos who was the final recipient of Roaille's espionage - he's known about Omega all along thanks to her (and, presumably us as well). The Ivy may be dead but the breadth her vines spread their bitter seeds are only now just being seen....
The website you quoted said that he was pretty malevolent - that final scene in 3.57 of him on Ala Mhigo's throne, where that hapless centurion reporting on the devastation wrought by Omega and Shinryu's battle, the look he gives the centurion where he dared to ask if they should devote some of their forces to reconstruction, resulting in the centurion very hastily backpedaling and making himself scarce, all but shows that. We've seen legatee do this repeatedly, killing their own troops on the slightest provocation after all (remember Nero and that Ala Mhigan infiltrator? The poor guy didn't even get a drumhead court martial, he was just killed outright on the spot.)
And besides, correct me if I'm wrong but didn't even Yoshi himself actually state that Zenos is a "monster" who "should not be sympathized with"? If his own creator outright states at how bloodthirsty and monstrous he is, then I think that's pretty conclusive that Zenos is nothing more than he appears to be, a dangerous antagonist that will probably have to be put down eventually for the greater good.