Pex = Polyethylene cross linked. Same stuff used in radiant floor heating systems.
The slipper spring configuration is an economical version of a leaf spring because it eliminates the articulated shackle and hardware used on typical double eye leaf springs. Presumably the springs are less costly to make as well.
I have slipper springs on my boat trailer. They do make a little noise from time to time, but usually the only time I notice it is when maneuvering over the berm at the end of my driveway. My driveway slopes down from the road so there is a 3 or 4 inch berm to keep rainwater from running down out of the street. As i make the turn out of the driveway I will usually hear a slap or "sproing" when the second wheel drops off the berm.
A little grease or slipper pad (delrin or reinforced nylon) between the spring and its wear surface, and the tubing slipped over the retainer bolt sound like good ideas to me.