So, I would like to ask for help to make the legend in the map print layout consistent as the legend shown in layer panel. I use the latest version QGIS 3.10.2 64bit.
This issue can be split into two parts. The one regarding the "broken legend-lines not aligning" I will adress below and which is already fixed, and the second one regarding NULL values you can read up here: How to make symbols visible in legend in Layout?. The second one seems to be partially intended (I strongly disagree this should be intended though: What if I dont want to display points with nodata, which is a quite common case?), but however, there is a "workaround" provided on GitHub and GIS SE.
The part with the "broken legend lines not aligning" is already fixed. See https://github.com/qgis/QGIS/issues/34166. I recommend to update to QGIS 3.10.12 or later to fix this issue. Alternatively you can use 3.8 or below, or as a poor workaround, if the above is absolutely not an option, create a screenshot of the correct legend and add it to the print layout as image.
This is a known issue but there are no perfect way to solve this right now. And it is partly intended. Just set a size when Null as per https://github.com/qgis/QGIS/issues/39882#issuecomment-723669783