1. Well, the mechanics forbid such an event from occurring. Your only resource are the cards in your hand. And you can either use them to increase your strength, decrease the strength of your enemy or draw a card (various versions). There are very few exceptions and they aren't overpowered. This philosophy, combined with the fact the basic removal, available from the start (the card Scorch), is literally one of the most powerful cards in the game, makes for a game that is more about out-bluffing your opponent and about tactical choices. There is of course some RNG (the initial hand) but you can mulligan 30% of it, that limits the amount of randomness you can have. This makes for very few overpowered cards. Of course there are always card combos that, if you don't have a counter (which most likely be some form of Scorch), can be extremely overpowered and almost always win the game, but they require 3 or more cards to play, so that's a big investment considering you only get 10 cards at the start.
To sum it up, there are very powerful combinations in the game, but not single cards. They can be answered but if you don't have an answer in your hand, then that's it. Thankfully it's a best out of 3, so even a slip up in a single round won't ruin your entire game.
2. Not really. That nomenclature comes from Magic:TG and was transplanted into HS. Gwent has an entirely different mechanic. Here there is no tempo, since there's no mana. You don't develop things on the board, on the contrary the best tactic in Gwent is to not place anything on the board, but that can be tricky.
There will be different types of decks and different strategies, but I doubt they will be called aggro, temp, mid-range and similar. Control, maybe, there are a lot control cards in Gwent. We'll see how the community develops. The one mechanic that is already in play and many decks may use, is Stall.