Deleted Lightroom Mobile Library

The trick for dealing with filled-up cloud storage in this case is to go to Classic and look at the "all synced photographs" folder. In there you will see all the images that have synced up to the cloud and have downloaded into Classic. What you do with those is to unsync them, which is done by removing them from the all synced photographs folder or by clicking the little syncing badge (the back and forth arrow) on the images in Classic. This removes them from the cloud, but keeps them in Classic. This in turn should remove just those photographs that have already synced up from your phone and create some extra space to sync up to the cloud. Your phone should immediately sync those and you go through the same exercise until they have all come over.

Basically this is what you were doing before but instead of deleting the images on your phone you should remove them from the syncing in Classic. This ensures that you always have a copy in Classic and you never delete an image that hasn't already synced over the cloud.

No workaround that I know of. Deleting means deleting everything on their server. While some of us have asked for a trash feature, Adobe haven't implemented one and don't give you any control over whatever backups may happen on the server.

Before you deleted the mobile content, did you allow any files to sync down into your Classic catalog? This would give you some/all of the photos that had filled up the space. What I would have done at that point is not delete - I would have unsynced those photos. This frees up space, allowing the phone to sync more photos until it fills up your allocation. So one might have to repeat these steps a few times.