GetMem, the author of OPM, does not answer his mails (I hope nothing bad has happened to him...). In the meantime I have volonteered to take the role of the OPM mainteiner. I still don't know much about the inner details of this software, and thus I cannot answer your questions with high confidence level:
/1/ rollback function: I've never seen something like that, and the OPM repository does not store older versions of the packages either. Therefore: no rollback support, you must take care of backups/previous versions yourself.
/2/ Partially installed packages: I could imagine that if you find partially installed packages this could be caused by the installation of another package which has the partial package as a requirement. When that required packages consists of several "sub-packages" very probably only the required sub-package(s) are "installed" (using quotes because in case of runtime package there is no installation at all since the IDE only store the path to the package in order to find it when needed). In principle, there is a compatibility risk when different packages are interdependent, not only with partial installations.
/3/ Cleanup: According to
https://wiki.freepascal.org/Online_Package_Manager#Cleanup_local_repository, this removes any unneeded packages from the "local repository"- this is the directory into which you download the packages from OPM. Usually this directory is in the user profile folder of a Lazarus installation. But if you use the same locale repository for several Lazarus installations (like me) then this feature could remove packages needed by another Lazarus version.
/4/ Update packages or wait for next Lazarus version: Packages usually are independent from the Lazarus version, and what I wrote about compatibility risk applies here as well. Therefore my advice (as author of many packages and current OPM maintainer): Use third-party packages only when needed, do not install just from a "nice-to-have" wish.
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