Open the web subtitles in subtitleedit. Change format to ass (advanced substation alpha). Save and re-embed using mkvtoolnix.
Positioning of multiple lines works well with ass and VLC shouldn’t have an issue reading and displaying. Not sure if YouTube includes the positioning data in their subtitles though. You could recreate that in subtitle edit (free software btw, dk web domain I believe) but it would be a bit of an annoyance.
edit: Corrected domain name. Not German, but nikse is it as OP has suggested
Yes. It’s incredibly powerful but easy to use for a basic purpose like editing a line or converting formats.