1. If the mod is mine? Yes go ahead fix away. My mods include source code so people can do what they want. I don't encourage "mod theft" but if the modder has abandoned his project, can't be reached, or died. Then others have been known to step in and repair or upgrade the mod and give the author of the original version credit.
2. The best way to decide this is to view the number of downloads and read any reviews. Ex: Go to downloads -> GTA VC -> Mission Scripts at the bottom of the list choose Downloads in the "Order by" box and Z-A in the "Sort by" box. The top dog seems to be "New Vice City 2004" with 92,900+ downloads. I better check it out.
3. No. My brother plays VC more than I do. I doubt he has either.
4. Not applicable.
5. This really depends on the types of mods. You can't use 2 banshee based mods, one would have to be altered to be used on another vehicle. Just the same you can't use 2 total conversion mods. Merging mods that use the same files would be difficult but not impossible.
6. Yeah well that goes without saying. Because if they never see it the modder can't really complain. Bottom line is modding can't and never will fall under copyright laws. No modder is ever going to take someone to court. I feel "mod theft" is just pathetic and everyone gets to point and laugh. If for example, someone said they modded a new freeway system for VC. I might think they have just ripped off the one that already exists. However if they used that one and altered aspects of it, they could add their name to the creds stating what they changed. But it remains a collective effort not their mod alone.