Can I really use one single firmware file for the various regions that my Sterling-LWB or Sterling-LWB5 module is certified for?

Yes. From release 8.5.0.7 on the same firmware file can be used in the various regions the Sterling-LWB or Sterling-LWB5 module is certified for.
Even though firmware file for different regions still exist and are avaialble for download, they are only there for legacy resasons. Either can be chosen.
Of course for regulatory compliace the regualtory domain still needs to be set accordingly.