Well, that's snapshots for you. Why people persist in using them escapes me. If you want a restore point, make a backup. Put a 2nd drive in the PC if you want to do the backups conveniently.
Ok, typical snapshot related disaster recovery mode: download the CloneVDI
tool from the Windows hosts forum. As the location implies, this is a Windows tool, so you'll need to borrow a Windows PC or install Wine to run it on a Linux host.
Copy the base VDI and all the snapshot files into one folder. Use CloneVDI to clone the most recent snapshot file (do not make the mistake of cloning only the base VDI, as the clone will contain nothing since the first snapshot). The output of CloneVDI is a single VDI which is a merge of everything from the most recent snapshot backwards. Build a new VM around the clone VDI - do not attempt to mount the clone VDI in the old VM, at least not unless you know enough about xml editing to delete all remnants of snapshot use from the settings xml file.
We know that the file contents are suspect - at least 2 files are corrupted from what you have said - in which case there is no guarantee of success with CloneVDI either. The only technique that guarantees success is a proper backup.