I would like to try out a previous build of 1.8.
Can I install a previous version of ROCK without reinstalling it? and can I somehow install an older version for iOS?
If you have a copy of that version of ROCK, you would need to reinstall it from scratch and then load a backup of Roon Database that was taken using that version of Roon.
If you don’t have such a backup, then you have an empty library.
No, on the latest version is available to download from the App. Store.
Can I ask why you wish to do this? If you have an issue with the latest build reach out to Roon’s support team.