The police will almost certainly be able to provide evidence that the request was entered into the postal system, properly addressed and paid for. Once they do that, it is assumed to have been "served" two working days later unless the contrary can be proved.
The time to provide the driver's details is now passed. The police must begin proceedings for speeding (assuming that was the underlying offence) within six months of the alleged offence and they will not entertain a nomination at this late stage. Whoever you nominate will have 28 days to respond and that will take it beyond that six month limit. In any case, once they have begun court proceedings they are most unlikely to rescind them.
That leaves you (or rather the company) with the choice of either pleading guilty or defending the charge. I would guess, especially in view of possible explanations (3) and (4) on your list, the prospects of successfully defending the charge are slim.
