If you have VSS enabled in a drive where a live Firebird database is working this is enought to corrupt the database as you described in the examples.

Also, antivirus softwares do this but ussualy only when the database became more than 1gb.

We had various problems like yours. We "solved" by performing an obrigatory backup and restore cicly every day and check if all is correct. When we have a problem the system stops, the client call us and we instruct (again) to do not use vss or a external backup program or file copy or antivirus.
This reduced our problems and always found a problem related with user configuration. We had only 2 corruptions related to a bad ram and a bad disk.

The next step will be detect if an antivirus software is running and tell the user to add an exception to our application. Also we will not run when vss is enabled.