Disclaimer: I don't know anything about Unraid, so my question might be stupid.

Assuming Ubooquity is launched through a script by the plugin, wouldn't it be possible to use a variable for the working directory that would allow the user the same flexibility as an explicit command line argument ?

Something like:

cd $MY_UBOOQUITY_WORK_DIR
...

I'am asking because using the "user.dir" setting is not a completely reliable solution (hence the bugs you encountered). So if there is a real need for a working directory setting, I'll implement it in the code itself. But I really want to be sure that this work is required as for all use cases I have been told about (except perhaps yours, hence my questions), the "cd into another dir" method was enough to solve the problem.

You're right, there is a problem with the webadmin.cred file.
I will probably fix it. However you can easily (and in a more reliable way) ensure that all files will be created in a specific directory by going into this directory before launching Ubooquity.

For instance if Ubooquity.jar is in /home/pi/apps and you want it to create its files in /home/pi/data, just "cd" into "data" before calling java:

cd /home/pi/data
java -jar /home/pi/apps/Ubooquity.jar

Much easier and safe.
Let me know if you have a use case that can not be solved this way (really curious).

Yes it's on my todo list, not at the top yet, though.

That's the bug I was talking about (the log directory problem).

You can do that using the "user.dir" Java option. Well, almost, there is still a small bug which will be fixed in the next release.
See details in this thread.

(as for the reverse proxy command line argument, I'll put it in my todo list)

Since the rewrite is going to take some time, here is a workaround:

1. Create a new theme (or use an existing one, as long as you can edit the pagereader.css file).
2. Edit the pagereader.css file of the theme you are using and add the following section at the end of the file:
   

   #displayedpage{
      width:100%
   }    

3. Restart Ubooquity after having checked you are using the theme you just modified.

After this, your pages should always use 100% of the width of your screen, no more, no less.

I have not tested it on mobile devices though (lack of time), so let me know if you encounter problems applying this workaround.
(or if it works for you, interesting to know too :))

It's a good workaround.

Don't bother too much though, I have implemented the solution suggested above (simple image file in the directory) as well as the user provided info through local html files in the folder.
They will be part of the next release (no date yet though).