It looks as if embedded client is used by PVD because fbembed.dll cannot be deleted.
You may have a point. I'm able to delete that with PVD running (connected to server).
I run Firebird as a service now, but until I figured out how to use it, I ran it as an application. Doing so provided a notification tool that would allow me to see if a connection I attempted was successful. If you're not already doing that, it might help you trouble-shoot.
What version of Firebird are you using? I don't know why there would be any problem using 2.1.2, but I'm using 2.0.5—because the embedded version used by PVD is 2.0.3.
I know nothing about Maestro, but this caught my eye:
With the trial version of Firebird Maestro I was able to find the option for indicating connecting to an embedded database, and then I was able to connect just fine.
Is it possible you have Maestro configured to connect to the database via Firebird embedded, instead of the server?