Since those above didn't answer this, yes, this is how it works. You can make commercial maps out of the box with the standard license. and those maps can be utilizing any of the assets distributed with the program. Note that this is limited to publishing the maps you make with said assets commercially, you're not allowed to distribute the assets themselves. But please use the contact details provided by Sue above if you need official clarification, as most people here can only give you an unofficial answer.

There are no specific assets for this kind of map that I am aware of.

There are some extra textures and symbols for Cosmographer through the two HiSpace issues in the Annual Volume 6. None of them are exactly what you are asking for, but they do give some additional options, including worn, dirty and rusty stuff, which can make it easier to approach making the kind of maps you want. I find those textures very helpful when I don't want shiny new stuff.

For sharing, yes, it is completely fine. The only things that matters when sharing the file is that the recipient have the actual art files on disk. You can make as many fills as you want inside the map without that being a problem, sharing issues only becomes an issue if you manipulate the png's on disk (such as making copies of them). As long as you can see inside that fill style dialog that both versions point to the same png files, then everything is fine.

The technique Ralf showed here unfortunately only works with bitmap fills. This covers most maps, but some maps, especially maps using line-work (like the landform maps) use different techniques for the fills.

It gets a bit more complicated to scale these. I am pretty sure Ralf showed that off in a video at some time too, but I don't remember which off the top of my head. I'll see if I can find a link for a video or something else, unless someone bets me to it.

If you try with for example the Mike Schley overland style, you'll find that this trick works the way Ralf showed it if you use it on a bitmap-filled polygon, like farmland for example.

edit: Ralf demonstrating editing hatch styles in here: https://www.youtube.com/live/-zZGWKT2pN4?si=LOm5YLc6CtJAds-j&t=1187

If you look at the prompt while using a mirrored polygon drawing tool, you'll notice that it offers you to hold down the control key to create smooth nodes. This allow you to create a plan that has both curves and straights in the same poly.