1. That's highly variable. The length of the platform is typically related to the length of the longest train stopping there, which is again dependent on the number of passengers in peak periods. But I've also traveled to subway stations that are actually to small to accommodate the entire train, so they usually announce on the PA system that if you want to get off, the X rear doors won't open, so start moving. But going with a length approximately similar to the train length itself is a good starting point, at least based on my own experiences.
2. I'm not an expert on that and don't fancy running the calculations myself, but this wikipedia page has some interesting data and formula: https://en.wikipedia.org/wiki/Minimum_railway_curve_radius
3. There is both a dimpled texture (but perhaps not close enough, and it is black) and some nice black&yellow hazard stripes in the @Bitmaps\Tiles\Storyweaver HighSpace folder

You'll have to :CC2EXPLODE: the symbol, that gives you access to the individual parts of it.

I have a blog article about X'es that might help you:

You're using the wrong version of the installer. The one you are using is the one for CC3, not for CC3+ which is what you have. You'll find the right one on your registered user download pages.

Not sure why you're referencing DRM here though, a technical issue like this doesn't have anything to do with DRM. This is just a compatibility issue where CC3 add-ons can't be used with CC3+ and vice versa, not a protection scheme.

I have a couple of videos on creating symbol catalogs which may help:

vthumb(dRRa6ctWPJY|Creating Symbols - Part 1 - Finding and Preparing Images)

vthumb(cYxAIPa7dak|Creating Symbols - Part 2 - Creating a Symbol Catalog)