I don't see any reason why they have to be 10' wide, unless they at some time had to be widened to that point to bring large objects through. In the real world, quite a lot of underground passages are narrow things, not even 5'.

You'll need to contact support by email, and they'll help you out.

Once you've entered this command, it is the new default until you set a new value, it is not a per-map setting. And it is a good setting for everyone to use unless you have an old computer with very little memory (below 8 gigs)

No, trace won't work on entities on frozen layers

When CC3+ renders a map to an image, it does so in sections. It does this because there usually isn't enough memory to calculate the entire image in one go. This value controls how many pixels each such pass should contain. The default value is on the lowish side when it comes to modern computers, so increasing it allows us to export the drawings in fewer passes, and the issue you were having usually occurs on the edge of each pass.