Yes, the .db files are the same on all platforms: Windows, OS X, Linux, Android and iOS.
You will need to get their Skype files onto one computer, merge them
there, and copy the resulting file back.
Select one database on Skyperious main screen, click "Compare and merge" and choose the second database. In the comparison tab, you can click "Scan and report" to scan for new messages in the first database and overview changes before merging, or you can click "Merge to the right" to scan and merge immediately. Either process will take some time, as every message in the first database has to be processed.
Afterwards, copy the updated file back to its Skype folder.
This can be automated with the command line interface.
! The databases should belong to the same Skype account.
! Shut down Skype before starting, if one of the files is currently used by Skype.
Yes, in source code form. See the installation guide.
Not quite. If one of the devices is logged in all the time, it
will keep the others in sync - mostly, but not always.
Merging the databases from time to time seems the simplest option.
As Skyperious can be used from the command line, that part can be automated.
Not directly, Skyperious does not work with *.dbb files. It is an old proprietary format
that was in use prior to 2008. However, Skype can automatically convert them
to the current format. Try the following:
No, it only works with Skype.
If the file is an SQLite database, for example a WhatsApp message store, Skyperious will still open it. You can't browse chats or export messages, but you can access, search, and export database tables.
I might add the support, if the requested functionality seems useful, and more people contact me to ask for it.
CSV by specification does not support non-ASCII letters, so they get converted to question marks. Export to an Excel spreadsheet will keep all characters.
SQLite usually wipes the deleted data clean instead of rebuilding the file
on each change. If you still want the file size to go down immediately,
VACUUM in SQL window.