I’ll have to look into / ask the others on this one - I’m yet to delve deep into Docker’s configuration/permissions options.
I’ve covered the process for this previously, but it’s rather painful, and requires building a custom QGC version.
Definitely - that’s one of the main kinds of use-case the Very Generic Indicator mini-widget is intended for 
Do note that the extension already provides a /widget endpoint that Cockpit can display in an IFrame widget, which includes some basic control over the data logging process.
Yes - it should be reasonably straightforward for the extension to output regular NAMED_VALUE_FLOAT MAVLink messages with the thickness estimates, if that’s a desirable feature.
You could raise an issue and/or submit a pull request for that to the Extension if you think it’s valuable
That’s another potential path, although I don’t see particular value in doing so unless you want more functionality than the extension’s existing /widget endpoint provides and you would prefer to use Node-RED for personal changes over making adjustments/contributions to the extension itself 