I have used no cleaning applications, and regularly back up my entire MacBook to an external hard drive via Time Machine. Backups to iCloud occur automatically.
You are aware, that the Time Machine backup will not save your photos, because you are using "Optimize Storage"? Time Machine does not backup the originals of photos, that are only in iCloud and not downloaded to your Mac.
If the copy in iCloud is your only copy, you have no backup of the photos at all. So you need to occasionally use "File > Export > Export original file", to save new originals to a folder on an external drive. Whenever I import new photos to Photos, I am saving the originals on my backup drive for originals. Since the originals should never change, saving them on Import should suffice.
3. KEY POINT: I would like to be able to access my photos in iCloud and restore selected ones to my MacBook (preferably into Photos with unique filenames) so they are detectable by the printer driver. I DO NOT want to have to download all my photos from iCloud
The download from iCloud should be automatic. When you double-click a photo in Photos to enlarge it and go to edit mode, the photo will automatically be downloaded from iCloud, if there is no problem with your internet connection. Try to enlarge a photo and go to "Edit" before you try to print it.
You did not yet tell us, if you checked, if "Photos > Preferences > General: Importing: Copy items into the Photos Library" is still enabled or had been turned off for a while. Please check this, if you have not yet done so. The error message you are citing suggests that Photos is expecting the originals to be stored locally and not in iCloud. And photos you imported while "Copy items into the Photos Library"is disabled, cannot upload to iCloud, but will be stored locally, outside he Photos Library and must not be renamed or moved.