1) No-it's perfectly possible to form an opinion on a game without finishing it. Very, very rarely does endgame content sway my opinion of a game, and the fact that a game can't keep my attention long enough to finish it is damning in and of itself.
2) I wouldn't say it's the primary goal, but it is a result. My goals in my reviews are always to express opinion-if all I wanted to do is tell people what they should buy I would just say-"It's awesome", "It's OK", or "It's sucks".
3) Journalism as we know it has been redefined so that it doesn't necessarily come from formal media (i.e., respected blog sites such as GameCritics), and with that we've seen lots of good, insightful writing from sources we wouldn't have otherwise seen, and tons of crap from complete morons. The definition of "press"
(look at the 31st) refers to "any media agency", which in today's world can mean just about anything. In this sense, GameCritics can certainly be considered journalism even though we aren't a formal media outlet.
4) A critique
can be considered any sort of thoughtful evaluation, so unless the review is totally devoid of any sense of thought I think they can be considered the same thing in many cases.
5) See answers to 2 and 4. I don't really see a Twitter post conveying that much insight into a game's worth, and again, GC produces plenty of "formal" reviews.
6) I can only speak for myself, but I could care less what the Metacritic score is-I evaluate the game as I see fit and if I'm out of sync with the rest of the Metacritic reviews then so be it.
7) We've all seen the Kane & Lynch incident, so yes, this does happen.
8) Yes. It helps if the author states where he's coming from in some cases to give the reader some perspective, but by and large readers shouldn't need to be familiar with the author's other work.