Sorry to hear you have so much troubles, but hopefully you'll manage to sort it out.

When you say wiped 40% of your map, make sure to check out things like sheets and sheet order. You may end up with something on an unexpected sheet after ungrouping, which means things may just be hiding out under something, like below instead of above your landmass or other terrain.

Not for lines. Polygons gets a hollow middle whenever the line width is greater than 0, but lines will be solid all the way, unless you use the "hollow" fill style (You're not, since your rivers are using a bitmap fill) OR you have an Edge, fade inner effect on the rivers, where the inner opacity is set to be lower than the outer one.

Looking at your drawing, you do have an edge fade inner which is way wider than your rivers which is also the partial cause of your rivers being very difficult to see. I didn't notice that immediately when I looked at the map earlier because I worked with effects off to see it without the glow. You can try changing the width of this effect to something like 0.2 and see if that works for you.

For that map, line widths less than 1 seem to be a bit to small to be visible, especially when zoomed all the way out. Using effects like that is a fine way to make lines more visible, or you could bump the line size a bit. I am guessing that doubling the size of the rivers should be fine.

Unfortunately not. A workaround is to access the system menu from the icon in the top left of the window and pick the save to clipboard option. Then you can paste it into your editor of choice and zoom in/enlarge it.

It depends what you are going for. To create a top-down overview, any city style should suffice, you'll probably be mostly using the house tool to create your own shapes rather than using finished building symbols.

If you're looking to map the interior, dungeon styles will allow you to do that. I think you're probably be best off with a regular dungeon style, the Castles add-on will probably not give you too much for a magic scool, other than perhaps some castle drawing techniques, but in general, you'll probably enjoy a modern bitmap based artwork dungeon style more.

If you are after the perspective views, then Castles may help you a bit, but it is probably better to start out with Perspectives 3 instead. But a perspective view of such a building is going to be a complex task, so not a good starting point for an inexperienced user.