Anything is possible.... it is probability that seems to get us one way or another... How long was the grinding allowed to go on? How much has the trans been operated in it's current condition of distress?
As another has commented, any diagnostic process is going to begin with "basics".... we begin with the least intrusive, simpler checks and tests and gradually work deeper - much like a doctor.
According to the WSM, the G5M-5 trans has a synchronized reverse..... Can the shift lever be moved into the reverse gear position? Are there any abnormal noises? If so, when do they occur? How many miles are on the car?
> The reverse gear on my 2006 Ford Fusion 5 speed manual transmission will
> not engage. This happened after a problem occurred while parallel parking.
> I had the transmission in reverse, then moved the shifter to put it in
> neutral and let out the clutch. But the transmission did not go into
> neutral. The engine did not simply stall as if it was in reverse, but
> there was a dreadful noise like gears grinding. Now the transmission will
> not go into reverse at all. Is there any hope of getting this working
> without tearing the transmission apart.
>
> --
> Mike Walsh