1) This is true, I shouldn't leave every time, but MC is more powerful than assassin's blade or BoK. I could double BoK a creature then you MC. Anything I do to that card throughout the entire game is mana spent. So lets say ive spent 15 mana on one creature to cast it, keep it alive, buff it up, even protect it with other minions. If you consider protecting the card as mana cost, which I would, then the total spent on it would skyrocket. Then you cast one card and take ALL that spent mana for yourself. In effect a 20 mana "valued" card would be purchased or stolen, however you wish to look at it, by the MC.
Saying that MC is more powerful than Assassin Blade is like saying that Onyxia is stronger than Silver Hand Recruit; yes, it is, it costs twice as much mana (roughly) so it's going to be a lot stronger.
Mind Control is an overpowered card (unless the opponent runs no big monsters, which is possible but unlikely), but it's an overpowered card in a class that has under a 50% win rate; i.e. the class as a whole is weak despite being propped up by an overpowered card. It's important to learn to play around MC; that's how you learn to beat it. If you quit everytime someone plays MC, then you'll never learn to beat it.
And that's my point with things like Assassin Blades. Cards like this can be game enders too, especially if they get Deadly Poison on it, since that's 20 damage to your hero that can't be blocked without taunt or weapon destruction. The point is that these are cards that the class is likely to play, and players still manage to beat them a lot, so it's a matter of you have to learn how to deal with it.
I'll say with Mind Control, by the way, that your strategy of using your big minions is part of your problem. You're talking about protecting/buffing your big minions, but that's the type of thing you DON'T do against a Priest who still has Mind Controls left. You actually want to do the opposite, and attempt to make your big minions "damaged goods", as I call it. That is, for example, if you have a Giant on the table, try to chip away at your own Giant's health a bit. Priests don't have much in the way of direct damage, so making it be at 5 health is unlikely to result it in being killed when it would normally survive (possible, but unlikely); if they're going to destroy it, it will usually be with either a removal spell or they'll steal it. If you make your big minion's "damaged goods", it reduces the value of what they steal and makes it easier for you to kill off. Of course, you can't do this the turn you play the minion usually, so if you play a minion and it immediately gets stolen this won't work. But often you will have the opportunity to have your big minions on the board for a turn or two, either because they don't have mana to cast MC that turn or they have other priorities. This will be doubly true when MC becomes a 10-mana card since they won't be able to MC it then heal it.
But the last thing you want to do is make some big, pristine super-minion that's all buffed up and such. Frankly, against most classes you don't want to do that, because that makes your target incredibly vulnerable to things like Assassinate. But this goes doubly so with Priests since you're just giving them a better target to MC. Remember, they're going to mc SOMETHING, and they're probably not going to waste it on something weak unless desparate; but you can still minimize how good any one minion is.
On a side note, you may notice that skilled Paladin players will do things like put Blessing of Wisdom on a Silver Hand Recruit (not against classes that can ping for 1 damage of course). And you may have wondered why they don't do this to their big minion on the table. It's for the same reason. Making a single super-buffed big target makes it an obvious target for removals and silences. By putting something like a Blessing of Wisdom on a token, they often force the player to divert resources they wouldn't normally waste on a little 1/1 to ensure it doesn't keep drawing, and beyond that, if they do silence/remove the big minion, it's one less card that gets destroyed in the process (the blessing that is).