Insert a pex segment with sharkbite/ push-to-fit connectors in an existing [copper] line

Try pushing the pipe in a bit, before pulling it out. Sometimes the barbs dig into the pipe, so the release ring can't disengage them. Pushing the pipe in, may allow the release to completely disengage the barbs.

1. Use the tool to compress the release ring.
2. Push the tubing deeper into the fitting, while keeping pressure on the release tool.
3. Pull the tubing out of the fitting.

During step 2, you might feel the release ring move slightly closer to the fitting. This is a good sign that the barbs are fully retracted, and the pipe is ready for retraction. If you don't feel the release move, don't worry, it might just be too slight for your sore fingers to feel.

Alternatively, try reversing steps 1 and 2. Push the tube deeper into the fitting, then use the release tool to operate the release mechanism.

I doubt you need the second 90 (#3) - PEX is bendable at large radius, and if you work with that, you avoid fittings that may have freeze damage (PEX itself does not mind freezing.)

If you think of it as if it was rigid pipe, you end up with a lot of fittings. If you work with its strengths, you can avoid many of those fittings, but you don't get tidy pipe-like runs.

I would be concerned with complete drainage ("slightly angled down while running under floor joists" may work for properly hung rigid pipe - pex is not rigid pipe.)