sonos arc position

Unfortunately, 19 feet is too far to hear the height channels when playing Dolby Atmos content. Ideally, with a 7.5-foot ceiling, your main listening position should be 6–9 feet from the Arc.
 

Would I be better mounting the Sonos on the wall or on the TV unit?

Placing the Arc on the media console would be the better option since it gets the Arc closer to your main listening position. Although I doubt this will get you close enough since your sofa is 19 feet away.
 

If it is on the wall, how high off the ground should it be?

With a 7.5-foot ceiling, about 2 feet from the floor will work. Make sure you keep enough space between the TV and Arc so there is enough clearance for the upward firing drivers. Sonos recommends at least 5.5” of space between the TV and Arc.
 

I have seen videos about angling the speaker to gain a better sound.... is this the case?

If you tilt the Arc forward, it will change the angle of the upward firing drivers which can increase the distance of where your main listening position needs to be. But it won’t make much difference if you are seated 19 feet away from the Arc.