To me the law is quite clear. 14.9c says the player could not be tackled from the direction he was.
9. Any player who gains possession of the ball :
a. Must play it immediately, by moving away, or by passing or kicking the
b. Must remain on their feet, and not go to ground at or near the tackle
unless tackled by the opposition.
c. May be tackled, provided the tackler does so from the direction of their
own goal line
Regarding the tackle area.
The tackle ends when the player with the ball moves away (Law 14.11b) but it doesn't say how far.
Move away is a phrase that is used frequently, but not defined.
The best thing we have is the definition of "Near" which is within one metre. This is used as below.
14.8. Other players must :
d. Not play the ball or attempt to tackle an opponent while on the ground