Select two layers at once (board outline and a layer you want to export) then click "Export Layer" button.

We aware of the problem but this seems a Wine issue. The same executable works differently on Windows and Linux. All we can do is to recommend stable version of Wine and 32-bit version of DipTrace even though you have 64-bit system.

Thank you for the report, I see the same. We will investigate whether it's a problem or not as changing layer type to mixed doesn't forbid traces on the layer.

Hello Allen,
Please check design rules. The clearance rules are too strict - 6 to 20 mm. With theses rules interactive router can't even start routing. Once you change the rules to more realistic you will be able to route traces.

How did you get slot holes in PCB? I mean did you use pads with oval holes or shapes on board cutout layer? In the first case, do not forget to turn on "Pad Holes' when you export STEP. In the second case, you should get the same result in STEP file as you see on 3D preview.

Via stitching and via shielding tools don't place vias on pads because often this is not good practice to do. But you can place vias on pads manually and connect the vias to the same net as the pad.

Thank you for the report, we will investigate it.

What issue do you have with design rules. Please provide some details, it will help us to verify the problem if exists.

Pads within the same pattern may be connected with internal connections. For example, pads with the same pad number become internally connected automatically.
There are options for internally connected pads. You can right click on any of internally connected pads, select "Component Internal Wire ...