It’s possible to choose the line spacing by default so that its parameters are applied in every opened document?
It's possible to choose the line spacing by default so that its parameters are applied in every opened document?
No (unless you write a macro that would modify every opened document).
Documents that LibreOffice opens usually (except for most basic formats like plain text) contain all formatting information inside. LibreOffice is not a “reading” application which goal could be to present text in the form most comfortable to reader; it is a document authoring software, which goal is to retain all original data, structure, formatting of the document which it contains. If a document defines a specific line spacing (and any complex document format, like ODF, OOXML, or binary MS Office formats do contain that, even if in form of using defaults defined in relevant standards/specifications), then LibreOffice must use that value, not some user setting.