In reference to the small buttons, you know you can change their size from :TOOLSICON:?

I don't think it is. The options regarding this is found in Tools -> Options -> Editing Options, but Tab is ind of an override for the default options, it doesn't actually change the options set here.

I would consider one of these approaches here:

1. Join paths to polys and create a multipoly of your floor before putting it into perspective. The disadvantage with the multipoly approach is that it is difficult to edit after the fact.
2. Split up the original trace into separate polygons for the rooms and corridors. This gives you a lot more surfaces, but it is easier to handle and edit. This approach will require a bit more processing for your trace before turning it into perspectives though.

The issue that occurs here is that because of memory constraints, CC3+ cannot compute the entire high-resolution map in memory at the same time for the export, which is why it exports in multiple passes, each pass rendering a part of the map.

Unfortunately, effects from entities outside of the current rendering part won't be considered, such as shadows from entities far away. There is a certain overlap to handle the closer but right outside things, but considering effects from everything in the map is impossible due to the aforementioned resource problem.

However, you can change the default size of these render chunks, causing CC3+ to render much more of the map in one go. This is done by typing in EXPORTSETMPPP on the CC3+ command line and hitting enter. The command line will then ask for the number of pixles, type in the value 40000000 (The current value, shown in brackets is probably 4000000, keep your zeroes straight). and hit enter. Depending on your export resolution, you will export the map in much fewer passes, perhaps even just a single pass.

Note that if this isn't enough, the other solution is to either consider using shorter shadows, avoiding the problem, or manually creating them using a partially transparent polygon.

To hide things from your players, I would consider putting things on different layers, then you can reveal one layer at a time as they explore the area. You'll have to predetermine which chunks to reveal of course, but this is a good idea revealing things piece by piece.

For elevation, as JimP suggested, contours are the best. Since you are using Mike Schely Overland, there is a contour tool called Height Contour in the :CC2MI: catalog. It looks white int he preview, but it is transparent, and you can lay multiple on top of each other to lighten it bit by bit.