Sorry to hear about all that! Unfortunately I'm not very surprised to hear you're having so much trouble with a bargain-basement system, though this is one of the worst cases I've heard of.
When you called Yoko, you were probably calling a factory in an industrial area, not an office set up to deal with customers. The reason that the manufacturer doesn't want to deal with you is twofold; they probably don't have anyone in their support department that speaks English (manufacturing companies in China and Korea typically only have English speakers in their sales department), and also because manufacturers generally only deal with wholesalers and distributors, not end users like yourself (end users generally have to go through the party who directly sold them the DVR for support). Think of it this way: if you buy a Ford Taurus that turns out to be a lemon, calling Ford's main factory isn't going to get anything done, you have to call the guy who sold you the car. The same basic model applies here. Just so we're clear, I'm not trying to explain away Yoko's product, I'm just giving you a little window onto the back-end of the surveillance industry so you can see why you probably won't get any help from the manufacturer. My point is that if the guy who sold it to you isn't helping, you're basically on your own.
That all said, to answer your questions in turn:
(1) I was unable to find anything through Google.
(2 \ 3) It won't be possible to change the software. Standalone DVR software runs off the system's motherboard, not the hard drive, and is tied very closely to the chips installed on the board itself. Even if you were able to change the software on the board (which can be done with the right equipment), chances are it wouldn't recognize the hardware and wouldn't work at all. You're pretty much stuck with the software you have.
(4) You might be able to find one on eBay if you were exceptionally lucky, but I doubt it will be possible.
Bottom line: There really aren't many options available to you to get that system working like it should, so you're really going to be better off just cutting your losses and purchasing a new, professional grade 16ch DVR (like a PC Witness Pro V\RT:
http://www.cctvwholesalers.com/product_p/dvr-vrt.htm). Systems like the one you have there now generally only get harder to work with, not easier, so I don't think there will ever be a point that you'll be happy with what you have.
PS - I edited your post to remove the direct link to your email address for your security. You should never post your email address on a public forum like this, since you'll almost always end up getting overwhelmed with spam and junk mail if you do.