There is a tutorial on page 94 of the User Manual.

For the map size, it is simply a case of multiplying the number of hexes with their size. Just keep in mind that hexes are wider when measuring corner to corner than edge to edge, so you need to know what those 24 miles of yours actually represent.

Have you remembered to add the Wall Shadow, Point Light Finalize effect? Lighting doesn't start working until this effect have been added.

This is because these maps were made in CC2, before things like effects. So to have a outline on the text, it is actually two text entities with the exact same text, one with regular filled text, and the other set to only show the outline. This was done to make the text more readable.

However, this means that you will need to do the edit twice, once for each text entity. If you have trouble selecting the right one for edit, try hiding some of the TEXT layers (either everyone marked OUTLINE, or everyone NOT marked as such.

As Wyvern said above, it is better that you start your own topic if you have any follow-up questions that are related to your own work and not the atlas repair project.

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.)