On further reading it seems that deleting the downloads.irdata folder is probably a very bad idea as the image files stored on HD by LRC are treated as the master copies and thus deleting downloads.irdata would cause all my cloud images to be deleted too. Would it work to simply uninstall LRC, delete all the mobile download folders and then re-install LRC and re-sync it with the cloud? There must be a solution here!
Don't do that (it wouldn't work anyway)! For future reference, the best way to delete images from both LrC and the cloud is to do it from LrC (if you delete images from any of the cloud apps, the original copy in LrC is merely unsysnced, not deleted). But deleting synced images from LrC will also remove them from the cloud....but if you prefer to do the culling from one of the cloud apps, don't delete them, instead just flag them as rejected. That reject flag will sync to LrC, where you can filter for the rejected photos then delete them from LrC, and thus from the cloud as well.
For now, though, you have to try to find all the images in the Mobile Downloads folder which are no longer synced if your goal is to delete them completely. One way to do that would be to select all the images in that folder (both synced and unsynced) and choose some way to mark them, such a by assigned an unused colour label to them all (though you could use other means such as flags or a keyword).
Let's assume for now that you don't use the Blue colour label in your catalog, so after selecting all the images in that folder (and using the Grid view, not the Loupe view) then press the 9 key on the keyboard. That will assign that blue label to all the images in that folder (make sure that "Selected Photos" is highlighted near the top of the Metadata panel).
When that's done, select the All Synced Photographs special collection in the Catalog panel, clear all active filters, then use the Library Filter Bar, Attribute option, and select the Blue colour label. That will list all the images in the Mobile Downloads folder which are currently synced with the cloud, so then pressing the 9 key again will remove the Blue colour label from them (they'll all disappear from the Grid because they no longer match the filter criteria, so then turn off the Filter).
Now if you select the Mobile Downloads folder again, and again filter for all Blue-labelled images, the Grid will then only contain the unsynced images. At that point you can select them all and delete them from LrC if that is your preference. Just be very carful to make sure that you are only deleting the unsynced images from that folder, and that is what you wanted to do....there's no "undo" after deleting images from LrC.
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