What you don’t seem to understand is DOCX is not a Writer native format and Word is not Writer. When you open a DOCX file, it is first converted to som internal representation which can be manipulated by Writer.
Then the document already contains many manual manipulations in Word and this contributes to a “difficult” conversion as demonstrated by the tons of one-of-a-kind styles trying to mimic forced formatting.
Not at all. Word is rather deficient style-wise. It knows only of an equivalent of paragraph styles. All the rest is done with manual formatting and this is where things become complicated: Writer must “reverse-engineer” individual formatting bits to rebuild a more global picture.
Word and Writer are not substitutable tools: it is the same has handling nails with a screwdriver (or driving screws with a hammer).
In the attached document, you show in the screenshots a tab defined at offset 0cm (not at 3cm as you claim). Level 2 is defined with a hanging indent of 1.5cm (thus “bleeding” in the left margin). The screenshot shows a tab stop defined at 0cm. This is usually nonsense, except in the case of hanging indent. It has not been retained in Writer for whatever reason (I don’t know why) but this does not matter because a negative First line indent implicitly defines a tab stop at the left indent (at 0 cm). In the end, you get the same behaviour as Word.
Regarding a tab stop at 30mm, I don’t see it in the screenshots. Are you sure it exists?