You can insert an image into your map using Draw -> Insert file. You can then either place this on a sheet below the one you draw on, or add a transparency effect to the sheet holding the image if you wish to have it as an overlay.

CC3+ has a command called TRACED than can construct an entity from an image automatically, but this works best if the image is black landmasses/transparent seas, and it will only be able to handle the outline, it can't really do inland contours and things like that (unless you are able to extract those manually as a separate image to be processed.)

Yes. There are a couple of ways to go about that. All the options do require some manual work to get a good end result, but getting the coastlines is quite simple (Although you do probably want to add additional detail to a zoomed in part of the world). See here for one way to do it

I can't make all potential useful topics sticky, the front page would just be a long list of stickies, which is not very user-friendly. Instead, it is listed in the resources category, which you can find directly from the left-hand navigation menu (Helpful resources and FAQ link)

Today I am adding Pappy's Pub by @Shessar:

And here's Sue's Town Hall:

Personally, I've just used them as visual aids and planning aids (but then again, that's what I use most of the maps for). Isometric isn't the best for battle maps, but at least in my world, battle maps are in the minority anyway, illustrations and planning tools are much more important, and for that, isometric gives a very nice easy to understand view of the place.

One I used recently was an outside view of a ruin