One easy way to tell if the piston is bent is to remove it and roll it on a smooth, flat surface. I have heard of bent pistons but generally it's after modifying the gas system for caliber conversions etc. If the piston is straight you almost certainly have a misalignment issue. Maybe someone more well versed on the platform can advise you better, but I think you hit the nail on the head when you tried it and felt resistance. Maybe loosen the assembly and straighten it the best you can, tighten everything backbup and see if it slides.
For what it's worth, I ran the calculations and the piston guide seems absolutely necessary to prevent buckling of the piston when under compression.