Blacksheep,
A significant positional error entered into the IRU (comared to the last recorded PPOS) will cause the align light to flash, prompting the crew to re-enter the a/c position.
If for some reason the crew were able to enter a wildly erronous position (longitude - I suspect you realise that the entry of a large error in latitude will not allow the IRU(s) to align) then when entering the flight plan, a route continuity (or similar) message will appear as the departure airport will not be the present location.
Even allowing for this, there would be nothing to prevent the crew from selecting ATT on the IRMP, following the system failure, to restore the horizon ball and entering a/c heading to restore the compass rose. It appears from the information we have however that this was not (or could not) be carried out.
P.S. check your pm's