Consider I have two OSGi bundles with the same symbolic name but a different version. Both bundles export the same unversioned package (e.g., my.package). Makes OSGi any guarantees which bundle will be used to provide the package? (A reference to the OSGi specification would be helpful.)
No.
Two packages with the same name and the same version are considered equivalent, so it shouldn't matter which is chosen. Therefore OSGi is free to choose either. See OSGi Core specification R5, section 3.6.4.