Looks like the granite is a retaining wall. Check with city hall about current height restrictions for retaining walls. In some areas, if you alter it, you may have to bring it up to current code.

What are your plans for the hot tub and fence?

The sky is the limit on these kind of projects. The lower part of the center wall looks to be brick with a parge coat. The left side has mortar between the joints and the right and far left looks to be dry laid. There may be a foundation for the brick but I doubt that the other sections have one. Depending on your budget and the availability of a qualified mason this is what I would consider. Repair and re-parge the center, make sure there is drainage, remove the mortar on the left and clean that up, replace some of the stones on the right , there are too many small ones and it doesn't look like the rest of the wall. Install planting for partial screening of the wall. Put the fireplace on the outside of the brick patio because you don't want to disturb the existing wall but if you got a lot of money tear it out and start fresh. All the best.