This measurement cannot be given without having the TV mounted on it and seeing how low it needs to hang. The best option is to mount the mount and TV and see if you like how it fits in the space. Best Buy does offer a 15 day return period to exchange.
If you have the space you should be able to use this mount without any issues. You should be able to tighten up or loosen certain points on the mount to keep it stationary.
This mount is only capable of mounting a TV, no other equipment can be mounted on this mount. There may be other mounts you can purchase in addition that will allow you to mount additional equipment.
That is going to depend entirely on the size of the TV being mounted. This mount can accommodate a TV up to 22 inches. Mounting a larger TV or a TV with a larger bezel will result in less clearance to the cabinet.