You can either buy right once, or buy cheap, buy cheap again, then buy right. Read Thom Hogan's tripod article for reference: http://bythom.com/support.htm
Back to the original topic, the 85 1.8 and 1.2 are two totally different lenses and really shouldn't be compared for anything other than portraits. The 1.8 is a nice lens and is better at nearly everything else, but the 1.2 is clearly the best portrait lens. For some it's worth the extra money for the seemingly small gains (or not so small as neuro's sample's show), for others, the 1.8 is more than sufficient. The 1.8 in a talented photographer's hands will beat the 1.2 in a rookie's hands, but give that same person a 1.2 and they will never want to go back to the 1.8, at least for portraits.
I agree with the points you made before, those things are all also considerations. I'll only quickly comment on them: I own a few of the Samyang lenses and I've used the rest. The build quality is generally very good. Samyang is a reliable company in Korea, you only need to contact them in Korea. Samyang covers their lenses. Samyang also licenses their lenses to other companies (Samyang manufactures them always, no other company is allowed to manufacture them, but other companies put their brand on the lenses, so you have Bower, Rokinon and many others) and if you buy the Bower or Rokinon lenses, you can contact Bower USA or Rokinon USA for warranty issues, they seem to both offer warranty.
If you want AF, go Canon or Sigma (don't listen to the focus issue scare, never had a problem with Sigma AF. My Sigmas are perfect copies and from what I've heard, not that many people actually have issues, the ones that do just talk the loudest. Anyway, Sigma will fix them for you if they're miscalibrated). If you don't need AF, most of the Samyangs are just as good an option in terms of IQ, buildQ and warranty. There are lens profiles for all of them that correct the distortion perfectly, but if you want a less distorted image to begin with, you're better off going with Canon on the 14 and 24mm.
Now for the rest of it: I agree that the f/1.2 can give you a little more than the 1.8, the question is if you really need/want more. The 1.8 is perfectly capable of delivering great results in the right hands which you admitted. If I didn't already have more than one great 85mm portrait lens and if I didn't need the faster AF for capturing action (meaning I can't replace the 1.8, I'd have to add to it) maybe I'd consider it, but the fact is that at the moment I'm trying to sell some of my lenses, not buy more. I have some 20 primes that are mostly redundant, so I have no room for more lenses before I clean up my "lens collection".