Q: unable to downgrade iOS 10 beta to iOS 9.3.5

Ask this in the developer forums.

to restore your device to factory settings see this article

Reset your iOS device to factory settings - Apple Support

otherwise for specifics or questions regarding beta software they can not be addressed here.

I know that you say you are using Apple's instructions, however does this support document contain the instructions you are using for downgrading? About iOS beta software - Apple Support

Go back to the current version of iOS

If you're using iOS beta and want to go back to the current version of iOS, follow these steps:

1. Make sure that you're using the latest version of iTunes.
2. Put your device in recovery mode and restore it. If you turned on Find My iPhone, enter your Apple ID and password after the restore to activate your device.
3. If you can't finish the restore, get help.
4. After the restore is complete, you can set up your device from your archived backup. This backup should be from an earlier version of iOS.
   

If your device has a prerelease version of iOS and you restore the device to an earlier version of iOS, you might not be able to restore your backup. This can happen if the backup is from a later version of iOS. For example, if you restore your device to the current version of iOS and try to use a backup that you created using a prerelease version of iOS.

And as others have stated, this is the most help you can get on a public forum.

The beta can not be discussed in the public forums. You should never install beta software on your primary device. It is intended for testing and development, not daily use.