Fixed defensive artillery is usually outclassed by mobile platforms.

Just imagine - a fleet of battleships sails in under cover of darkness and bombards positions at first light, annihilating coastal defenses. Then and only then are the transports sent in toward the beaches... but then, that's what happened.

No; the counter to an amphibious assault should be aircraft. The landings at Normandy were made possible by Allied air superiority. If the Luftwaffe had been given increased priority for training and production but was placed under the operational control of area all-theater commanders (who might possibly have employed German naval strength in conjunction), the situation would have been entirely different.

Partisans can be suppressed over time. Air-dropped infantry is vulnerable to armored troop concentrations (see Market Garden). And no ground force can long exist without supply. Destroy the ships or the artificial harbors and you destroy the entire invading force.

Yes, front-line armor would have made a difference; personally, I prefer von Rundstedt's 'defense in depth' to Rommel's plan, but either would have been better than both.