Is there a way to point LibreOffice extension to a LanguageTool server instance? That would ease configuration when running a server instance anyway. As an example, ngram data usage has not to be configured for two independent LT installations.
Sorry, that's not supported.
I see that the Firefox extension (only) supports client server mode. But then, no-one would write a 400 pages book in Firefox. So, is it not yet supported or is it intentionally not supported (e.g., to avoid huge traffic on public LT servers).
It's not yet supported, but I don't know of anybody working on it or planning to work on it. Firefox only supports client mode as it's technically not possible to run a Java-based software like LT inside Firefox as an add-on.