This happens because the background is a fill without scaling configured. This means it will repeat to fill the area depending on the resolution, so a high-resolution export will have many more pixels than on-screen display, thereby causing to repeat more.

To fix it, you'll need to click the fill style indicator in the top right, go to the bitmap files tab, find the relevant fill in the dropdown, and then check the "Scaled" option for the fill, and provide proper values for the X/Y size of the scaling. You'll need to experiment with those values to find the correct setting, but once you've found something that works on screen, since it now is scaled, it should then be the same size on exports.

That's just how copyrights works unfortunately. It's still their material to do what they want with even if the products isn't available anymore.

Even if WOTC doesn't keep the patches on their site any more, they're still available using archive.org though. I just tested the links, and they work fine.

After changing them (I recommend taking care not saving over the originals, and keep your own modified version in a separate folder with a slightly edited filename). You can then import the symbols into the map using Symbols -> Import PNGs.

After that, you can open the symbol manager, and find your imported symbols (they will be at the bottom). Select all the new symbols, and hit the save as catalog to save it as a new symbol catalog. Now, open the new symbol catalog as if it was a map (instead of into the symbol catalog window), go to the symbol manager again, but this time, select the symbols and hit properties. From here you can set up options like symbol groups and random selection from groups. Just save the file normally when done.

From now on, you can load this new symbol catalog into the symbol catalog window when working with a map and use it like any other catalog.

Here's Wyvern's Travelling places of Embra, now in the atlas.

I still have two sets remaining in my backlog from these before being done getting all the Embra maps into the atlas.

Wyvern's Hilly Places of Embra are now in the atlas: