Files deleted by pressing del key

When you delete a file into a sandbox, it still is available from outside Sandboxie, because the delete is only performed within the sandbox. An exception to this rule is, when the folder is set to "Direct File Access" or "Full File Access" from within the configuration of the sandbox. Then the delete will be performed in the real folder.

You run an installer sandboxed and install a program, then you create project files with the program. You delete files into the sandbox.

Are those files still available from outside Sandboxie? No, because they were created inside sandbox folder and they didn't exist previously outside.

That's right. My description above was meant for the case, when the file is existing on your harddisk outside of Sandboxie, and from within the sandbox you delete the file. Then it can be accessed still from the outside.

When the file was created from inside the sandbox, and then deleted, it is really deleted.

The only chance of getting back such a deleted file could be, when you start an utility for undeleting files. This tool should search in the folder, in which the sandboxes are stored. For example, if your sandbox is stored on "D:\Sandbox\" and is named "DefaultBox", and your windows-username is "Fred", so the root path to the sandbox is "D:\Sandbox\Fred\DefaultBox\". In this folder you can see the directory "drive", followed by the path structure, in which your deleted file was saved before deleting. Probably the undelete-Tool can recover your file from this place.

I think that's the answer he was looking for: use a file recovery tool inside sandbox folder.