Speaking of "ONE CHANCE" be sure you don't nick the surface of the PEX by letting the hacksaw jump when you start your cut. That's assuming you use a hacksaw. Lowe's should have a cutting tool for PEX that might not be too pricey. It should cut well in the middle of a run but I'm not sure how well it works when you want to only cut a slice off an end. I know the ones for PVC pipe do NOT do that well.
I was surprised that Big Box didn't have something that'd help you, sorry you had to make a second trip. I bought a nice all plastic shutoff valve that I cut into the PEX for our RV toilet at Lowe's, about $12.
From the pic it looks like you might have to cut a lot out and rebuild from where you can get SharkBite fittings to work. That or cut the PEX clamps and try to get the PEX off the brass without gouging it. If those barbs are damaged, I fear it'll leak.