To convert to the Finale 2010 .mus file format I suggest that you go “the MusicXML way":

1) Open the .musx document in a version that can read the .musx file format (= 2014 or 2014.5 or v25 or v26).

If you do not have any of these programs, try e. g. the Finale v26 demo.

2) Export as a MusicXML file.

3) Import the MusicXML file into Finale 2010.

The free Dolet 7 Plug-in for Finale 2009–2014.5 gives you even more MusicXML options and increases the accuracy in what you'll import from newer versions (especially lyrics and expressions). This lets you exchange files with Finale 25 & 26 users more accurately and with those who do not wish to export to older formats.

https://www.musicxml.com/dolet-plugin/dolet-plugin-for-finale/ 

You will be limited in your ability to edit features that do not exist in 2010 but you'll have a much better chance to see everything in scores created in newer versions of Finale the way they were laid out originally. 

As Peter already told you, the only way to make a direct conversion is from a newer version—and the demo works for 30 days (export is crippled afterward). Fortunately, the upgrade price from 2010 is the same as from any other version.

https://www.finalemusic.com/ 

Failed to import file MusicXML (Finale 2014) into Finale2010 ((

What happened instead? If you go through the steps you took, from exporting the XML to your importing (including which version of the Dolet importer you used), that will give us better info with which to help you.o

After using Dolet, the file opened, but with distortions. For example accolade missing, the notation of dynamics has shifted ... I also downloaded the trial version of the Final 26, there is something to compare ..
But thanks anyway for the advice ...