I have always suspected that a G17 barrel would fit in a G19 and protrude in front of the slide. If for example one had a need for room to add threads to the end of the barrel. Will they interchange?
Yes. This is often done when buying a suppressor.
Technically yes the mounting system for the barrels is at the base end under and before the chamber they are identical in form, would i recommend doing it no, the action of a recoil operated semiauto pistol is controlled by the mass of the slide and spring tension, but for the first few of mm of travel the slide and barrel recoil locked together then the barrel pivots down to unlock and allow the slide to continue rearward cycling the action, the added mass of the longer barrel will most likely mess up the harmonics of this cycle, changing the recoil spring to a slightly lighter one would probably eliminate this problem though.