The guns not firing is most likely an engine limitation. The engine can only track 64 objects at once, and that includes gun blasts, fighters, asteroids, engine plumes, even the nav point marker. The wcdx patch does nothing to address this (or exacerbate it).
As for graphics upgrades, any sprite replacement that works with the DOS version should also work with this version, but you can't actually increase the game's resolution; the only way to do that would be to write a whole new graphics engine. WC1 just won't support a higher resolution. That said, there are lots of high-res fan-made models floating around the CIC somewhere, and I believe there was a Prophecy mod a long time ago that recreated some of the WC1 missions.