This is a great price for canvas pliers:
* When I got a bad dent on a canvas once, I filled a spray bottle with hot water, and misted the back on the problem area. Worked great....you might try hot water.
* Golden makes a GAC 400 acrylic polymer...not sure if it would solve your problem if the canvas is poorly stretched, but you might check it out:
"GAC 400 - Stiffens textile fibers. Dries to a very hard and stiff film. Most useful for priming fabrics (cotton, linen, silk, etc.) when stiffness is desired. May be used as a rabbit skin glue alternative when applied directly to raw canvas or linen. Allows sculpting and shaping of draped fabrics."
* You could remove the canvas from the stretcher frame, and adhere it to a panel instead.