You can create a mirrored copy, just pick Mirrored copies from the right click menu of the :CC2COPY: button.

You may want to have snaps or ortho on when defining the mirror axis to ensure it is perfectly straight.

That's the standard for all images in the atlas. I export them without a grid so people can use them in VTT's. An existing grid grid in the image often makes issues. Since you cannot turn on/off the grid in an image like in the fcw, I find the gridless versions more versatile for the image (and no, I am not going to publish two different versions of the images on the website).

For users of the actual atlas, they can just toggle it in the fcw.

The square is still there because it wasn't on the GRID sheet. (Which is actually fine, because a visible square helps VTT users to tune the in-software grid.)

You should be well on your way with CC3+ and SS3. Do note that SS3 is a generic modern set, it doesn't have anything in specific for superhero, but on the other hand, most of the specific stuff would be the superheroes themselves anyway, the rest is mostly standard modern. Of course, no symbol pack is ever going to have every single symbol you're ever going to need, but these are a good start.

We currently don't have any other sets for modern available, but for Sci-Fi, there is Cosmographer 3. Note that the art style in Cosmographer 3 is a bit different from SS3 since they are by different artists. You can mix and match in the same map if you want to, but they were not designed with that purpose in mind.

There are some free community provided packs, and while mostly focusing on fantasy, some of them (CSUAC and Dundjinni archives mostly) also have some modern and futuristic symbols.

And, as Sue said above, you can use any .png file as a symbol, so you can find and add symbols from other sources you may have access to.

No. The issues in the sampler are complete versions of the issues included. The full annual would give you lots of other nice stuff, but nothing more for this style.

Not directly, it want a relative change.

But, you can get the desired result by just typing in the new size divided by the old size on the command line, as in, just type the actual calculation, no spaces. So if the old size is 3.7 units and you want it to be 2 units, when it asks for scale factor, just type

2/3.7

on the command line when it asks for the scale factor.

You do need to know the old size though, which can be found using the distance tool in the info menu.

This is commonly used when inserting an existing map to trace by for example, just insert the image, measure the distance of the scale bar in the inserted image, and use that as the basis for scaling.