From paint to photoshop - collaborative worldbuilding - advice needed

[Lawrencelot]

Hello all. This is my first post here so sorry if I do something wrong. On another forum we started a collaborative world-building game, where players add one country at a time, in paint. This has resulted in the following map at the moment:
colabwb10.png
All the countries have a name and backstory, and the white area is not yet claimed/explored. Now, for aesthetic purposes and also to be able to play a game (atwar) on the map, we were thinking of making a geographical map for this world. I tried some photoshopping techniques I knew together with the atlas tutorial to make this:
cwb_geo_labelled2000.png
Note: the labels are copied from something else, they are not done yet.
I was hoping I could get some advice on the following:
-How can I make the pixelated areas smoother?
-How should I tackle the rivers?
-I want to include land borders as well, but I'm not sure how to do this, as the edge function in photoshop gives all the mountains as well.
-I did not follow the atlas method for the mountains, but just used a standard photoshop texture, is this fine? I used the same texture for underwater mountains as for normal mountains but there might be another option.
-The lines in the south are supposed to be huge waterfalls. Any idea how I can change the paint version to a realistic version?
-The North Ring is supposed to be a mountain range of immense height, separating two worlds. Does this come across? Also, the ice world north of this range is supposed to be some kilometers higher than the main world, any ideas to get an effect for this? And can you see it's ice? Because right now it looks more like sky to me.
-The world is supposed to be doughnut-shaped, not spherical. Are the grid lines correct for this?

Hope you can all give me some feedback and help me out. Thanks for your time!