To understand how Osseum could be responsible for taking so much money from so many people, you have to realize two things.
First, some hiccups in the money supply are completely normal at the manufactuirer end. For them to get their money, the retailer has to pay the distributr who has to pay the fulfilment house (in a case like Osseum), who pays the manufacturer. So if the far end says to you "there's a bit of a snag, but I'll have it all worked out by next month, or at worst the moth after", it's not going to be the first time the manufacturer has heard this. If the fulfilment house has treated you right for three year, and you're his biggest client, there's no reason to think he's lying.
This is particularly true for someone like Pramas, who has been in this industry for a lot of boom/bust cycles. And the whole reason for going with someone like Osseum is that they'd proven to be a valuable service. Also, publishers tend not to talk to each other about problems too much, for lots of obvious reasons. Plus, it sounds like Osseum was paying something at first, which would make their story sound even more reasonable. And, of course, Osseum could tell GR "We only sold 400 books this month. Must be a slump. We can only pay you for 200 of them, because the slump is making distributors cash-poor." It wouldn't be until Osseum skipped town and people got into their warehouse Pramas would be able to look and see how many books were actually gone. An unsold book can be recovered and sold. A book that's just missing and you never knew had been sold is more lost money, especially if you're GR and you've been selling through your print runs.
Second, Osseum got all the money, but was only supposed to keep a tiny fraction. It was a business of margins. I don;t know exactly how GRs numbers worked, but in general terms if Osseum sold $6,000 of GR stuff into the distributors, Osseum was supposed to kep it's cut (say, $600), and pass the rest on. But that meant if Osseum was losing money, perhaps because it had grown too big too fast (I've had friends tell me they know of people who sent notices to Osseum mentioning they owed Osseum money, and Osseum was too overwhelmed to collect), or because of warehousing costs (Osseum's warehouse was so stuffed you had to walk atop pallets of books to get to the back), it would be easy to prop it up for months by stealing a bigger cut and claiming less was sold, or that the distributors haven't paid yet.
My bet is the owner thought he was having a "growing pain", and that if he could get extra money just for a month or two he'd make it all work out. So he stole money, and claimed everythign was still alright. But the core problem (whatever it was) didn't get fixed, so he needed more and more money to make ends meet. When he finally realized he'd stolen hundreds of thousands of dollars, and everything was going to heck and he couldn't save it, he was unwilling to face either the personal nor legal consequences and just left.
My guess, looking at the books in questions and John Nephew's articles on how much things cost to print and such, is that GR lost between $200,000 and $400,000, depending on the size of their print runs. That's not to say all that would have been profit -- not by a long shot. Most of it would have gone to paying the bills, which is exactly why GR ended up shorting its freelancers, as this thread mentions.
I know some freelancers freaked. I've spoken to a few. But it looks to me like most knew GR had been worked over by someone else, was being upfront about it, and was making payments as they were able. That's the way you handle a disaster like this, and it speaks strongly of their ethics and the quality of their products that GR is still with us today.
Now, I'm gonna go buy some M&M stuff, and more Advanced rulebooks.