I just finished Rick Atkinson's fantastic book The Guns at Last light, which is an account of the war on the Western front from the Normandy invasion until the end of the war. Using that as a source-

1. The Germans knew that an invasion was coming, it was a matter of where and when. There was a large Allied intelligence program dedicated to perpetuating misinformation to the Germans that the invasion was coming in the Pas de Calais, north of the actual invasion beaches in Normandy. The deception and misinformation campaign is a fascinating story in itself and is worth checking out.

2. That's up for perpetual debate-if the Germans had reacted quickly, and mobilized their forces in the Pas de Calais area for a quick counterattack, it's possible that they could have pushed the Allies back to the beaches and held them there. There's so many historical what-ifs attached to this scenario that it's impossible to say for sure. It took several weeks before the German general staff was fully convinced that the Normandy landings were the main invasion effort and not a diversionary landing, they were still expecting a larger landing in the Pas de Calais area. Keep in mind that the Germans were still able to keep the Allies bottled up in Normandy for 2 months after the landings, fighting a slow defensive battle.

3. The Normandy landings were very much an "all-in" operation-if they had failed it would have been a disaster for the Allies and would have likely prolonged the war for years. I don't know of a specific back-up plan but from what I have read the attitude of the Allied general staff was the failure in Normandy was not an option, and would have been an absolute disaster.

The coastal defenses are a fairly straightforward indicator that Germany took a landing as a potential threat. One of the hardest parts of a large amphibious assault is the supply chain and risking transports at sea. England made an ideal staging ground versus say Leyte Bay in the pacific. Knowing the allies were shipping tons of material right across the water, the German permanent defenses (coastal artillery, pill boxes, naval obstructions, bar wire, walls and men) let us know it was a highly considered possibility.

Naval power is an interesting story. The Anericans alone ended WW2 with over 6500 ships. Granted lots were supply and troop transports but we had plenty of brand new Iowa class battleships, cruises, destroyers and destroyer escorts to bombard with. However admiral King kept as much force as possible in the pacific to beat the Japanese and devoted most of the rest to U- boat defense. The real scare was the Allied Air power which could rain down equal devastation by rushing in quickly then getting the hell out.

Other plans- yes- kinda. It was dicey to open up the front but king and Marshall thought it was doable. The other option, and actually one continued to be pushed by Churchill after D-day was to continue pushing up from Italy. The Americans didn't really have the appetite for it though. In their mind after Rome there was no need to go further north and the dual front with bombing raids was more than enough.

1.Germany considered an amphibious assault a certainty. The timing and location were the only issues in dispute.

2.Static defenses will never defeat a mobile force. Rommel, who was in charge of the beach defenses understood this and wanted his armored forces near the beaches to drive the allies forces off the beaches back into the sea. He was overridden and reserves were held further back and released after beachheads were established and were harder to dislodge. But ultimately I don't think either decision would have changed the ultimate result of overwhelming allied superiority in materiel.

3 Operation Dragoon on August 15th 1944. The invasion of Southern France by another invasion force coming up from Italy. would have been significantly more than a footnote had Normandy failed to achieve its objectives. But understand that there were 5 separate landings in Normandy, plus 3 airborne assaults. Only Omaha beach was ever in jeopardy. (and that ,only for less than a day) Meanwhile Utah, Sword Gold and Juno went reasonably according to plan. Had Omaha failed completely, troops from Utah and possibly Gold would have moved inland and pinched it off from the rear. It would have been a Bitch but it would have been done. Allied forces were there to stay. Believe that.

Given how useful destroyers were in support of the D-Day landings and how, later, scuttled ships were used to form Mulberry harbors, where there any plans to use navy ships as artillery platforms on the landings? IIRC, the Japanese tried to use the Yamato as a such a platform on Okinawa, but it was sunk en route.

My question is: Why didn't the landing ships opened on the backside or the sides? You always seem to see videos or pictures of soldiers getting gunned down from the front. Didn't the engineers thought of side openings to provide some cover?