You say that, "Apart from the fact that the group of 17 members is highly unlikely to collectively own all patents that are essential to WebM...".
Um, evidence required. It's not at all clear that there are ANY other patents essential to WebM, other than the ones that Google bought and released. Yes, MPEG-LA claims that there are some, but they are NOT a disinterested party; their entire business model depends on extortion of funds for the use of ANY format. I haven't seen a strong analysis showing specific patents (with patent numbers), why WebM necessarily infringes, *and* why those patents will stand in court.