IIRC, it was Finale 2014 which started using the .musx extension, meaning that all files will (in principle) be compatible with all future versions, both backwards and forwards. Version 2014 also had the possibility to make files reverse compatible with Finale 2012.

… Version 2014 also had the possibility to make files reverse compatible with Finale 2012 …

Not only version 2014.

Also v25 can export to the Finale 2012 .mus “legacy” format.

I suspect that the coming v26 can export to 2012 .mus format, too.

Remember that up until 2014, there was no backward compatibility at all, except via XML. I wouldn’t be surprised to see the Export as 2012.MUS disappear as well, at some point in the future.

To expand on the above... If dealing with others using multiple versions of Finale, the 2012 .mus export files are not backwards compatible to earlier versions of Finale. MusicXML, however, is.

For maximum compatibility, those still using Finale 2009–2014.5 should download and install the free Dolet 7 plugin. This functionality (MusicXML 3.1) is built into 25.4 and later.

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

OK, now can I just get a simple straight forward answer? Can version 26 load version 2014.5 without error? Can version 2014.5 load version 26 without errors - or at least with easy to manage and/or predictable errors?  

Yes.

(Is that straight forward enough?)

Will the Dolet 7 plugin bring compatibility between V. 25 and V. 2012 files? 

Also, will that plugin bring compatibility between V. 2012 and earlier files?

Finale versions 25 and 26 have an internal export function in the File menu to save a file in Finale 2012 format so that one does not have to go through the XML process to transfer a v25 or v26 file to 2012. Of course some changes or defaults in v26 particularly will be lost/changed.

http://www.rpmseattle.com/of_note/finale-version-26-review/  explains some of this in its review which affect opening v26 files in v25 and 2014 versions as well.

What J Adrian says. 

Dolet 7 adds MusicXML 3.1 compatibility to Finale 2009–2014.5. Finale 2012–26 users do not need it to exchange files, however.

It’s still a good idea to have Dolet 7 if you exchange files with users of other notation applications. Sibelius, Notion and some others are on MusicXML 3.0 which is later than the version in F-2012. Dolet 6 would have brought 3.0 compatibility to older versions and 7 includes that as well.