Your commentary on the first diagram seems to lack a bit of understanding so I'll post an explanation below, plus I've never explained this topic on here before!
Speed or velocity is not a force so you can't compare it with friction to determine what will happen to the car. The tangential speed is constant but it is continually changing direction which means the velocity is changing and a change of velocity means that there is acceleration. This acceleration acts towards the centre of the circle and so there must be force acting towards the centre of the circle which is known as the centripedal force.
The centripedal force required to keep an object moving in a circle is
Fc=rmv2So this force is proportional to the square of the tangential speed. The faster the speed, the bigger force is required to maintain circular motion. This will make sense if you imagine yourself in the car.
In your first question since you're considering maximum speed, this is going to require a big centripedal force to maintain circular motion so friction will need to act down the slope to maintain this big force. So you have this situation:
The first diagram shows the three forces acting on the particle and the second shows the horizontal/vertical components.
The centripedal force must satisfy
Fc=Frcos(15)+Rsin(15).
So the friction needs to be big enough to maintain this big force directed left otherwise the car will slip. There is a maximum friction which means that eventually as you increase the speed there will be a point where the centripedal force cannot be maintained anymore and the car slips.