1st and goal from the 5 yard line. the offense has 2 people in motion at the snap, we will have a flag at the snap. the runner gets tackled at the 9 yard line. what would the enforcement spot be if the defense accepts the penalty?

For any foul that occurs at the snap(illegal motion, formation ectect) by the offense is enforced from the spot where the ball was snapped from.

how about this situation: 1st and 10 from B's 40 yard line, A player holds at B's 35 yard line, the runner is tackled for a 7 yard loss back to B's 47 yard line. where is the enforcement spot?

On a running play, the "basic spot" of enforcement is the end of the run.

In the play you described, the foul occured behind the "basic spot". We would use the all but one principle ("screw the offense" as I like to call it) and enforce from the spot of the foul.

If accepted, 1st and 27 at As 43 yard line.

I like to call it 'worst-case scenario' for the offense: Whichever is worst for them, the end of the run or the spot of the foul.