Nice Map! The somewhat muted colors makes is pleasant to look at, I like it. A few observations, some of which might be irrelevant:

Your main continent and the southern isle-continents seem all to be aligend horizontally... it looks just very similar to me.
I like that you distinguished all landmasses with a different color scheme, but compared to the main continent the outer areas have litle variety, or "green" zones.
Since there is no scale I can only guess, but there are very few rivers. Is that on purpose?

Regarding some of your questions:

3. I like the barrier tape line around the map, makes it look nautical. The cover and interior maps from Paizo's adventure "Savage Tide" have a very similar line, which you could try to imitate. Replace the black border with parchment maybe?
4. Dashed lines are easy, if you know how: get a rectangular brush shape, then go to your Brush Palette (F5 for Windows). Under "Brush Tip Shape", change roundness and spacing according to your needs. That's all.