Thanks all. Regarding the highlights, I often referenced this video which was useful for a lot of different aspects
https://www.youtube.com/watch?v=p6dtYdWPabI&feature=emb_title. Although having done a few more renders I'm starting to agree the highlights are overdone.
I'm moving the camera closer to get a few ISS style shots, the one above is shot at 300mm from far away which is probably making the scale a bit odd. The map isn't flipped, it's looking towards the east (sunrise) at the northern coast of Brazil.
To understand the intersection issue I have to give a bit of detail on the setup. I am using the NASA 8bit elevation for the positive displacement, and a NASA bathymetry map for negative displacement below the surface. But at 8 bit, neither map has enough vertical resolution to accurately define the coastline. For example in low lying land area, e.g. Florida, much of the landmass is still represented as 0 in the heightmap. There is also overlap between the elevation and bathymetry maps around the coastline, so I cannot raise these areas out of the water. As a work around, I have used another mask I found
here which better represents the coastline (and rivers), and despite it's lower resolution, it allows me to better define the coasts. So I use this mask to raise the land out of the water, and sink the seafloor, by a fixed amount in either direction. You would think you would only need to do so by a few metres... but for some reason I cannot avoid the artifacts in the image shown unless I make these offsets at least 100m (the effect of which starts to become apparent if I render close ups). Increasing the micropolygon detail also seems to help reduce some of the artifacts. I figure the issue is that I have the ocean sphere too close to the planet, the darker/speckled areas being where the two surfaces are too close (and these go away as I increase the offset beyond 100m or so).
I'm at a point where I have mostly solved any issues that are affecting the final renders, but if there's anything I can do to avoid the offset I have had to create along the coastlines it may solve headaches down the track
