Okay, I only read 2 pages, but I'm stating my opinion anyway (maybe someone alse already wrote what I'm going to write..);
Considering you work at a highly professional level with prices way over demo-bands normal budgets, I actually think you should have checked up on them good before starting. Sure they told you to proceed, but judging from their bad english you should've checked up on them real good. Their myspace looks like shit/sounds like shit/like.. 44k visits, no friends, they existed for 5-6(!!) years and achived this. I would NEVER start mixing a client like that before telling them prices. You REALLY can't expect a band to know how the mixing process works (for example knowing that it takes 3 days to mix a crappy sounding demo that's 9 minutes long)
Now, having that said, don't get me wrong. They really need to pay, you can't accept anything else! They are handling this very unprofessionally, childlike and wrong. But I'm just telling you this for the future. I read what James said, but you can't always relay on thinking the bands know what they're getting them self in to at this level (if the band is a demo-band that sounds like shit!).