SM78K

When simply wanting to know if a pin is High or Low, what should I do?

Activate the task called "78Kx Simulator GUI" from the Windows task bar at the bottom of the screen.
The output Settings dialog box is displayed when [Specify Output] is selected under [File] in the GUI menu.
Please check [Timing Chart] for output here.
Next, select either [Output Only] or [I/O] and click [OK].
The timing display window then opens. Specify the pin to be checked under [Set Pin Name] here.

I create programs by splitting them into multiple files with the assembler.
Since only one file can be downloaded for *.lmf file download in the SM78K0S, I can see only one file.

Only one .lmf file can be seen because all the program information is contained in the one .lmf file that was created.
Although I do not know the concrete operation being performed in your system, I imagine that you probably tried to open a multiple source windows?

Basically only one source window can be opened (source corresponding to current PC).
Maybe this is what you are referring to?

To open multiple sources, the manipulation method differs depending on the SM78Kx version.
In the case of the latest version, it is possible to open just the source you wish to see.

If the SM78Kx is not the latest version, request an upgrade from your NEC distributor.

If the SM78Kx you use is not the latest version, with one source opened, set the status where the window is not active by selecting [Operation]-[Hold], and specify the source to be opened by selecting [File]-[Open].
Several sources can be displayed by repeating these steps.

The displays of the timer compare registers (CR20 and CR80) in the SFR window remain -- without changing. Why is that?

This SFR is write-only.
Therefore, these registers cannot be read.

How can communication be realized with the simulator? Also, what is the input data generation method?

Either create a 01 pattern using the 01 Editor or create an input file by specifying the pin, data, and timing in the input timing chart window.
Download the input file thus created prior to simulation.
For the concrete method, refer to "Input Timing Chart Editor Window" in the Simulation User's Manual.
Other than that, you can create an external part and call the serial pin data input function as a function of that external part.