If you go to do a new install, you might want to use
MultiSH3 and create a new Save folder of a different name. The game will use a folder "C:\Users \UserName \Documents \SH3" by default (computer and UserName dependent), but with MultiSH3, you can have it use most any 3-character name you desire, such as "GWX" or "LSH" or whatever you want to use to be able to remember and associate it with a particular mod / game combo.
Since your current install won't run at present, I would experiment with the Properties settings first, and see if you can get it to at least run. Then deal with the rest, one step at a time. Base any new install off of what you learn about your system. The only issue I have with a copied Steam install with GWX on my laptop is if I try to use certain WideScreen mods, then Steam will seemingly "block" its functionality, but if I use standard SH3 resolution, and do the Properties stuff, all is fine with the world. Another of my laptops had no issue. Other folks have no issue with the Widescreen mods either. Just make certain you are using the full, and proper Widescreen mod for GWX. Some of what look to be full mods, do not have the full mod set-up, and are basically "updates" instead.