As I understand it, the lateen is a far more difficult rig to handle in heavy weather – thanks to the sheer length of the spar – and ships built for the North Atlantic (where the vast majority of Viking ships sailed) had to cope with heavy weather. I seem to remember that Spanish shipmasters in the mediaeval period would use both rigs, lateen for sailing the Mediterranean and changing over to a square rig if they went west of Gibraltar, and that suggests that the guys who really knew about it understood the problem! It also occurs to me that a lateen rig might be fairly efficient on one tack, but the fact that spar (and therefore the sail) must be constrained by the mast on the other, doesn’t make it as effective on the contrary tack. I’m sure the Vikings knew of the lateen rig, and possibly some longships were rerigged if they were based in the Mediterranean, but if the lateen had really been more effective in the Atlantic then, trust me, they would have adopted it! These guys knew what they were doing, and so did mariners in succeeding centuries and, again, the lateen rig didn’t spread to the Atlantic (except in a smaller form on the mizzen). So they didn’t think it was a better rig, and I trust their knowledge!
As for Greek Fire, yes it’s a stunning weapon, and those guys loved wonderful weapons. If they had known its composition and how to project it, trust me they would have adopted it, but they simply didn’t know! The recipe was lost, which is why it wasn’t used in the Middle Ages either. It isn’t stupidity, or a reluctance to try new things, it’s simple blameless ignorance. As for the arbalest, they knew about it, but on board a ship an arbalest would be incredibly clumsy, slow to reload, difficult to aim, heavy and generally useless unless your enemy conveniently stayed motionless!