WebM is also the name of the whole project, no? You're right in that VP8 is the biggest target, but Vorbis may also be a target. Matroska I doubt will be a problem.
How is WebM as a container format a minefield? I've never heard of any sort of potential litigation problems against Matroska-based containers, only the potential litigation issues pertaining to VP8 itself.
That is called uncertainty. Yes, it may now infringe on anything right now. But you cannot guarantee it will stay that way for the next 100 years. That is the biggest issue today.
They've been trying to pull this over for YEARS on Theora, and you know why it never worked? Because they never cited actual patent numbers. Never. The leader of Xiph.org has attacked them again and again and told them to bring on the heat numerous times if they believe there's an actual problem - which he doesn't believe there is, seeing as how him and many others have spent the past 10 years EXPLICITLY making sure there is no infringement on any patented technology of MPEG or anyone elses - and they have only ever said that they "believe it is open to patent lawsuit." You know what their solution is? "Just pay us royalties if you're using Theora, and in case any patent issues DO come up, you'll be protected anyway!"
They're trying to extort people on that front. They're known patent trolls. End of story.
On the note of VP8, like I said earlier, it's pretty probable that because VP8 and H264 are so similar in so many ways, that VP8 probably infringes on some of H264's patents. At the same time, like I said earlier, it's also pretty likely that H264 infringes on some of Google's patents which they acquired from On2 (some of which likely date back to the times of VP3/VP4.) There is no clear winner here, and if they DO decide to open litigation, what will they do if Google says "Well, we clearly have patents that H264 infringes on. Take a look." Essentially it's just nuclear deterrence, only with patents instead of nuclear weapons. Nobody wins. If it wasn't Google, sure, maybe MPEG would probably have no problem suing them into oblivion despite the fact they could have patents on you. Whether or not that tactic would work, against, uh, Google is another matter entirely.
Well, I might have been thinking about Microsoft. Sorry.
No, they aren't. The issue is that Mozilla has ideologies to stick to, and the main one is to keep the web free. H264 is not free. It never will be. That is why they will not support it, ever.