If you can boot via a USB stick, and get MorphOS to run, even if it is a basic ready-to-install version, then there will be a version of SFS doctor on the USB stick drive.

So, if you use SFSDoctor on the unreadable volume, then SFSDoctor will try to salvage/save/fix the volume, and I have had usually good experiences with salvaging HDDs using the program.

If you've got to the MorphOS screen then you can almost certainly try out SFS Doctor on the HDD volume and see if you can salvage it, by running SFSDoctor, even if you cannot access it directly from Ambient to save what you can before you try to salvage/run SFSDoctor.

Give it a try, if you can, as you don't seem to have anything to lose - well not more than you've already lost !