Quote Originally Posted by rotorgun
2. The German high command felt that enough transport could be provided by September to transport at least two Panzer and eight Infantry Divisions across the Channel in two waves. Logistics where planned to be delivered by air to the airfields captured in south England by the invasion forces. August was the deadline for the decision to proceed.
It's not possible to transport tanks across the Channel by river barges. It's not possible to transport men across the Channel by barges, unless the sea was utterly calm and the crossing was utterly uncontested. Neither the Channel nor the Royal Navy and Air Force were known for these behaviours.
3. I believe that a sortie by the Italian Navy in the Med, perhaps threatening Gibraltar,
There was already a fleet in the Med ready to deal with the Italians. There may have been a problem should the French fleet join the Germans, but the British took care of that at Mers el Kebir and Oran.
coupled with a faint invasion fleet aimed for Scotland would have drawn a good portion of the Home Fleet to sea. There they could have been dealt with by a combined air/submarine campain and crippled. This would have left an attenuated force with which to guard the Channel.
The Home Fleet sailing south from Scapa Flow would first be under the cover of 13 Group, untouched by the Battle of Britain, until they reached Tyneside or slightly futher south. Then they would be under the cover of 12 Group, which was barely touched by the Battle. Note that this route takes them close to the British coast and far from German bases. Only when they reach the mouth of the Thames do they start coming within reasonable range of German aircraft.

If the situation had been serious enough for the Home Fleet to be sent south, the bulk of 13 Group and the whole of 12 Group would have been used to keep them safe on the journey. A heavily armed convoy from Britain suffered 60% losses on its way to Malta, on a lengthy journey that was mostly within range of enemy coasts. The passage of the Home Fleet would have been a hugely more heavily armed convoy consisting entirely of warships expert at dealing with u-boats and accompanied by massive air support.
4. In a sense, a "dam" of sorts could have been created by the German surface elements guarding an invasion corridor sreened by as many U Boats as were available. The job of such a force would be to draw out any remaining RN forces to destroy them with the aid of the Luftwaffe, assuming that the RAF was Neutraliized by the earlier attacks. With a combined arms approach, I believe that the RN could have been driven off or defeated. It would have been costly to the German Kriegsmarine no doubt, but such a sacrifice would have been justified for Sea Lion to suceed. As soon as the RN was stopped, let the Blitzkrieg begin!
The Kriegsmarine tended to let loose their ships individually, so the British couldn't so easily catch them, and quickly, keeping their exposure in open waters to a day or two at the most until they reached the Atlantic. If they were afraid of sailing in convoys or dallying in the Channel for a few days at a time, what makes you think they would dare to set up a permanent stand there?

Also, the passage of Gibraltar was regarded as tantamount to suicide by u-boat crews. Wouldn't you think that with Britain threatened with invasion, the Channel would be even more heavily guarded?