1. Yes completely possible, this is something that people think are even happening to this day even though it probably isn't the case, as they should have already taken control of the the network if they really had this.

2. No, the network would know as they're hyperactive as to this sort of stuff happening. Would probably trigger lots of indepent miners trying to fix all of this and remove you of the 50 percent that you'd be holding.

3. Yeah, not going to answer this one as it's insanely far fetched. Sorry!