Check my recent blog article, it describes ways to fix Red X's.

Do note that moving the files to the installation directory is not really a good idea, the setup with the data files in a separate directory is for a good reason. Also, maps that uses the old relative paths (starting with # meaning installation directory) should also automatically be redirected to the data directory, so it may indicate that your map didn't use proper CC3 paths in the first place, so fixing the path sin the map is a better option for the future compared to moving the files, even though it may take more effort.

True. You don't need much snow before any resemblance of the roof structure is completely gone.

You can do mass replace from the symbol manager. Just pick the symbol to replace, and then pick the one to replace it with. Note that the symbol you wish to replace with already have to be placed in the map once, otherwise it won't be in the symbol manager list.

Note that you can only replace one symbol type at a time, there isn't a way to simply replace varicolor to non-varicolor.

It's mainly CPU-bound, but the speed of your drive (for loading artwork) and speed of your RAM will influence things. And while things like effects do take advantage of multiple cores, I also find it helpful to have a CPU with a high single-core performance. It's better with a handfull of cores and high performance per core, than a huge heap of cores with lower performance per core.

I find it a nice resource. It should install fine with CC3+, but the procedures do require some manual unzipping of some zip files, so it is important to unzip these exactly as described in the guides, as ending up with one more or one less folder in the path is guaranteed to lead to red X'es.....