The choice of trees is up to you, but something to bear in mind when borrowing things from other styles is that while you might love the look of the trees from an alternative style that doesn't mean they will automatically be the best match for the style of the map you are making. If you don't like the combination maybe try a few other styles, including the trees native to the style of the map. Similar styles usually work better together than vastly different ones, and it is usually easier if the two styles have a similar colour scheme.

It sounds like you are using a style which has separate coastlines. Look at your sheets in the sheets and effects dialog. If you have a sheet called LAND and a sheet called COASTLINE then to adjust only the coastline thickness pick the COASTLINE sheet and hide all the others. Then you can pick the coastlines and adjust their width and reshow the rest of the sheets.

A general note that might help in the future - if a polygon has any kind of line width greater than zero the fill will vanish and leave just the outline.

That looks a lot better with the bevel on the land too. How about using an Edge Fade, Inner on that sea contour to blend the two ocean colours together just a little?

Looks pretty good already :)

Have you considered making the textures larger? Sometimes the detail can be a bit too small to look like anything much. And don't forget that you can also use the same texture on different sheets with different colour altering sheet effects.

You could use a variety of styles that might reflect the culture of the map maker. These days most real world maps seem to look fairly similar, but that was not always the case.