That would have raised even more questions though wouldn't it?
Why would it have?
The trigger logically
shouldn't be BB-8/the missing piece of the map, otherwise we
should have seen some sign that R2 was waking up as soon as he came in contact with BB-8 for the first time.
I know this is the "official explanation" from the writers, but it sounds too much like misdirection to me, like
JJ saying that they gave C-3PO a red arm to show how much time has gone by even though his arm is restored by the end of the film, thus breaking that logic completely. (It seemed more like a symbol of what was going on with the other characters, with Han and Leia being symbolically wounded and Han sacrificing himself for his son, and then Rey becoming more of a whole person, all of it inextricably linked to a story thread they were careful not to reveal.)
Timing it with Rey's appearance seems to make sense, but we have no idea how R2 would be able to activate upon her appearance. A genetic scanner, perhaps?
The lightsaber would have been a physical object that wouldn't have come in contact with R2 at any point during the rest of the movie (when Finn had it, all he wanted was to save Rey) that would have signified that a person who inherited the lightsaber (and thus symbolically accepted the Force) specifically is ready to go find Luke.
Anything would be better than the situation we have currently, where people question the fact that it happened for any particular reason at all to the point that I'm willing to call BS on the official explanation from the writers because it's that bad and that unbelievable.
-Have we seen a Droid cold boot before?
Technically, no, but we don't see any signs from R2 whatsoever until the scene where he activates and he isn't
"cold booting" anyway since he's in low-power mode.
As simple as Luke have him a command to hibernate until a key arrives. The map being the key.
See the above.
suggesting that R2 is force sensitive? Could Ren have woken him?
If Rey is Luke's daughter/is already known by Luke, it could be a genetic key.
- Finn sure did get lucky to be the one to escort Poe
I'm pretty sure he was just bullshitting in that scene to get Poe out of custody.