Keeping an older version of the lib inside your addon will only work if no other addons or versions of the lib are loaded. Libraries in ESO are a shared global resource and it will always try to load the newest version in any case.
I can't speak for other library authors, but I am very careful to not break the api defined in a version of a library ever since I did just that a long time ago and someone complained about it quite angrily.
If I have to make breaking changes, I either add a new function in a minor version or do a major update which will provide a separate instance of the library object in order to not break addons that use the old version.