Remove 1.5” spa pipe fitting from PVC pipe WITHOUT damaging pipe

I wouldn't use the PVC pipe at all.

Here are some things to think about...

1) Splitting the PVC pipe in half will involve two saw kerfs that will make the re-mated halves not restore to original size.

2) PVC cement will not work well in trying to re-join the two pipe halves. That type of cement is meant to be used in tight lap type joints; not in rough butt type joints.

3) If you have to use the expanding foam you really should place the pipe parts in place and secured first and then inject the foam through one or more small holes through the pipe wall.

Overall I would suggest that you reconcept your approach to this and keep the lower portion of the walls as is but convert then to knee walls with height of maybe 40 inches. Temporarily support the beam within the current opening space. Then cut out the existing studs to produce knee wall framework. The top of this knee wall gets fitted with a good double 2x4 header. Trim the top of the knee wall with a flat trim board that is wider than the drywall on each side by some 1/2 to 3/4" and trim around the skirt of the board with a nice molding. Then set a partial turned 4x4 post on top the flat board and use it to support the beam. Finish out the top of the openings over to the side walls down to the bottom of the beam, band with drywall and finish. I think this would give you the better look and still open up the space perception. The shorter 4x4 type turned posts are much more reasonable in cost.

Excuse my somewhat less than elegant cut/paste/smear work with paint program but here is the general idea of what I am suggesting.

First, check the basics:

• Are you turning the right direction? It is easy to get backwards when you are in an unusual approach. Inspect the thread closely for the correct loosen direction.
• How long is the wrench? An 8 inch wrench is probably insufficient for loosening 1/2 inch copper fittings, though it probably is sufficient for tightening them. Try a 12 or 16 inch wrench. The greater leverage of a longer wrench provides greater torque, but more importantly provides more control at that torque so you won't be as tempted to "pulse" the torque.

To protect the valve and safeguard other plumbing, be sure to provide counter-force at the other side of the joint with a similar wrench or vicegrips, etc.

In my experience, WD-40 is no help with plumbing.