Let's take these one at a time.

You should be able to open 2012 files from within 25. If this does not work let us know but it should. Assuming that 2012 files open within 25. You can now run Get Info on a 2012 file, select 25 as the app to open it and check to make 25 the default to open all .mus files.

2012 doesn't work as well in Sierra as anyone wants. If Human Playback is selected, 2012 will crash when playback begins. This happens around 100% of the time.

I have lost my 2012 finale installing disks I would like to have one sent to me as I have bought it so I can update to a higher version, Thank you

You would do that through customer service, although in the past, they have always said that older versions were no longer available. However, if you go to your account, you should be able to download previously purchased versions.

But you don't need disks from an older version to upgrade.

I skipped 2012 so I don’t know if you could download it. I’m remembering a demo so I suspect that, like 2014, you can. 2011 and older versions were never available for download. In my MM Users account, I can still download) every version of Notepad I ever registered (not that I will ever use them again)

”But you don't need disks from an older version to upgrade.”

What MikeR says. In addition, with competitive cross grades etc., the upgrade process handles these things automatically.  

I see Finale 2012 listed under Previous Versions in My Account; it is the earliest version of Finale available for download.

Thanks for clarifying about 2012. I suspected as much but didn’t know for certain.