What is the best way to cut this PVC line in a hot tub?
I can see your predicament: cut it all or nothing. It is possible to remove PVC pipe that has been welded into a fitting. I've sawed out pipe sections from slip valves and re-glued the valve with no leaks. It's an extreme last ditch effort, but I don't see any other way other than cutting the entire manifold out . Hopefully some one else will see an easier way. Here's how I'd do it:   Looking at your first photo, the short length of pipe as it enters the 45, can be cut half-way up the hub  so that half of the connection is left (left on the 'Tee and short pipe stub). This will free one end of the housing. The amputated 45 sleeve attached to the pipe that is the inlet can be section with a saw and should peel off with pliers a segment at a time.  The outlet at the 45 and housing connection; the same can be done. This will free the housing. At the 45 you will have to cut segments into the still glued pipe. A hacksaw blade works well. Cut a minimum of 6-8 segments around the inside pipe and only cut the pipe not the fitting hub it is glued to. The more segments you cut the less chance of cracking the fitting when removing the pipe and the easier it is to remove. A quick blast with a heat gun to soften the PVC helps.   If you are successful you'll only have some minor saw marks after the pipe is removed. They won't affect the seal once new sections are glued. Use primer and "Hot" (blue) glue when installing new pipe.