After installing Windows 7, CHKDSK has ruined my Windows 10 HDD

One source of the problem is Fast Startup in Windows 10, which is basically a logoff + hibernation.

Windows 7 was unable to recognize that state of the Windows 10 hard disk, so it just proceeded to destroy it. Then Windows 10 booted and completed the destruction.

Anyone thinking of dual-booting Windows 7 and 10, should first Turn Off Fast Startup in Windows 10. Or else. Even then, one might encounter a situation where each Windows version will demand on each reboot to chkdsk the disk of the other version, which is of course risky.

Another problem was using incompatible user accounts, since any account created by one operating system does not exist in the other, therefore doing chkdsk with one version on files modified by the other is playing hell with the security IDs of the files.

The "solution" for the accounts incompatibility problem is to use only standard user accounts. A standard account that might be safe to work under is the hidden built-in elevated Administrator account, which needs to be enabled. However, Microsoft has good reasons for hiding that account, mostly for security.

If you do not have a prior disk-image of the Windows 10 disk, the situation is pretty grim. The only measure which might return some sanity to the Windows 10 installation is probably Repair Install Windows 10 with an In-place Upgrade. If this does not work, then a full reinstall is required, after formatting the disk.

Dual-booting these two versions of Windows is inherently a risky proposition. A safer solution would be to use Hyper-V under Windows 10 to create a virtual machine with Windows 7 installed. This would be much safer than dual-boot.