If you start the walk from Ferrol, you still walk 100 km and therefore if you take the short cut over the bridge from Ferrol to Fene, you should still obtain your compostela.
For me, although I did it, the walk all the way from Ferrol to Pontedeume, all the way round the estuary via Neda, was too long, and on the first day, it affected me later. If I did the Camino again, I would either stop on the first day at either Xubia or Neda (but this makes the first day quite short) or I would take the short cut across the road bridge from Ferrol to Fene and walk to Pontedeume via that route.
Staying at Neda does add an extra day to the journey, which some people may not have, but I agree with Faith above that the walk around the estuary is a lovely walk, and possibly the most picturesque part of the Ingles.
If you take the short cut, be sure to obtain a sello somewhere between Ferrol and Pontedeume (I tried, without success, to obtain a sello at Fene town hall, and others may have more success than me - it might have been the time of day - lunch, but I obtained a sello at Cabanas town hall). Therefore, on the first day, I had 3 sellos, one at Ferrol at the start, one at Cabanas town hall, and one at the end of the stage at Pontedeume town hall. Although you only need the minimum of 2 sellos per day, I always tried to obtain a minimum of 3. I did not obtain a sello at Neda because I could not find the town hall, and therefore anyone who says that you need a sello at Neda in order to obtain a compostela for the Ingles is incorrect. There were no questions asked about my route from Ferrol to Pontedeume at the Pilgrims Office in Santiago.
If you do take the N-651 road bridge across from Ferrol to Fene, you walk the Ingles until you come to the roundabout junction underneath the N-651. Don't worry, there are zebra crossings across the sliproads when you walk across. Although a dual carriageway, the N-651 bridge does have a footpath on each side of the bridge, separated from the carriageway by crash barriers, so there is a physical barrier between you and the traffic, and then another barrier between you, the edge of the bridge, and the sea. I can imagine though that the wind swell of trucks passing at speed just the other side of the crash barrier over the bridge could possibly cause an issue to those not steady on their feet. If you did the bridge short cut, I would walk on the Northern footpath, rather than the Southern one, because this will make an easier walk when you go up the hill into Fene (there are then less road junctions to cross on the Northern footpath). Basically, when you have crossed the bridge, you keep walking straight up the road and up the hill until you come to Fene town centre, whereby you cross the road which leads to the town hall at Fene, and at the end of that road, you rejoin the Ingles proper.
final point, the footpath over the railway bridge is a different /smaller shortcut than the shortcut over the N-651 road bridge from Ferrol to Fene, which I cannot imagine is closed, as this is a major traffic route out of Ferrol.
