When I try to launch any of 1.7.3-1.7.10 versions in Demo mode (as a non-premium player)
the game does not start at all and the standard message "Game crashed" is displayed in the launcher (see attached: "crash-message.png")

In the output log shown by the launcher I can see the following error:
Exception in thread "main" joptsimple.MissingRequiredOptionException: Missing required option(s) ['userProperties']
...

I attached screenshot of the log: "crash-log.png"
and full text log from the launcher: "crash-log.txt"

At the stage the error occurs, the game itself doesn't write any log yet (probably because it hasn't started fully yet), so I could not provide any log from the game itself.

However I analyzed the command line that is starting up the game and I think I know the reason for the problem.
I also have a suggestion how to simply repair the error.

The command line ends with:
(...) --accessToken --userProperties {} --demo
The "--accessToken" parameter seems to require a value for itself.
For a non-premium player trying to play the Demo mode there is no any value specified for "accessToken", so the parameter "consumes" any possible value, which is here just "--userProperties" text.
This in turn causes the lack of "--userProperties" parameter needed for the game, which throws the exception: Missing required option(s) ['userProperties']

As a confirmation of my supposition you can check the case of version 1.7.2,
where there is no "userProperties" parameter in the command line,
so the command ends with: (...) --accessToken --demo
Here the "accessToken" parameter "consumes" the following text (which is "--demo")
that causes the version 1.7.2 running in normal (full) mode (not Demo) for a non-premium player!
You can simply check it in the launcher, as 1.7.2 is the only version starting successfully for a non-premium player in Full version instead of Demo, which (as I suppose) is an unintended side-effect.

The exception concerning missing "userProperties" parameter happens also in version 1.8, but NOT in 1.8.1.
In 1.8.1 version the "accessToken" parameter still has empty value, however "userProperties" parameter does not seem to be required anymore, because "consuming" it does not matter, and as a result the game is starting up successfully (also with the "--userProperties" parameter completely removed from the command line).

A solution for all the problems I have described is simple:
the launcher should set any value for the "--accessToken" parameter for a non-premium player and all the errors are gone, including the (probably unintended) full mode access to version 1.7.2.