Those ball valves will probably outlive the house, so no harm leaving them. The only good reason to have them is convenience ...like now, during the remodel.
The valve needs to be attached to a crossbrace of some kind, to prevent movement in the wall, even though is sounds like a tight fit.
Hammer arrestors could be used. We don't use them much around here, but some people like to use them.
As far as placement of the valve, the installation instructions and that plaster guard fitting give you the specific placement of the valve with respect to the finished wall surface. If you don't follow that spec, you will have problems attaching trim and handles.