another question about riverland border defence. if the kings of the rivers and hills had managed to secure control of the golden tooth, would it protect against the westerlands as well as it protects the westerlands against attacks from the east?

It would certainly help: 

If the Riverlanders held the Golden Tooth, it would act as a bottleneck against any incursions down the River Road pass, holding back any attacks until the Riverlands had a chance to mobilize, and giving the Riverlanders a chance to fight the Westermen in the mountain pass where the larger numbers of the Westermen couldn’t be brought to bear, as opposed to the more open valley below where in OTL Jaime steamrolled over poor Vance and Piper. 

Now, it can’t prevent the Westermen from going through Deep Den down the Gold Road and then swinging north to attack the underbelly of the Riverlands as Tywin did in OTL, but there’s still a significant advantage in shutting down a potential route of invasion and knowing that there’s really only one route the enemy can take.