Interesting. So I am guessing the Germans would launch Georgette just the same, but there is no Operation Blucher but instead continued attacks against the British (no point in attacking the French because the British can't move reserves down there anyway and you would want to force the British to expend supplies in a supply limited situation.
Its hard to imagine the British just running out of supplies and surrendering though. They would improvise something to land supplies on beaches or improve the remaining land connection to at least allow a defensive fight.
However there would be no build up here and any eventual couter attack would have to be south of Amiens. The Germans would be in better shape because they would still hold their original lines along the Asine and any success they have up north shortens their lines.
Additional German success would delay allied offensives in Salonika, Italy and Palestine as the British would move all their reserves to france (south of Amiens).
Campaign extends into 1919, however the Germans are completely on the defensive and knowing their allies are shakey would leave divisions in Italy (8 divisions), Salonika (4 divisions) and Turkey (1 at the straits and 1 in the Taursus mountains) to prop up these theaters and since these areas are not great avenues for advance with the Germans defending them nothing happens there really (the Allies do take Syria and the rest of Iraq).
So all the main action is on the west front with the Allies reaching the Rhine at some point in the year, that with the threat of a strategic air campaign as long as the allies want and with the complete defeat of the german submarine offensive, the Germans ask for peace. The eventual allied victory will be seen more as an American victory as compared to OTL, perhaps this American peace might be more lasting.
The British problem was not port capacity, but rather rail capacity north of the Somme. Their logistics were very shaky and actually collapsed during the Somme offensive in 1916 until a civilian rail expert was brought in to reform the time tables. Still, there were a dearth of rail lines north of the Somme and through Amiens ran the only double track line across the Somme.
So the problem is that if Amiens falls the British simply cannot make up the necessary supply losses of losing access to their Southern Ports. This won't make then surrender of course, but it could cause a supply problem grave enough to force the British armies north of the Somme to fall back on their ports to ease the strain. They can hold out in bridgehead-like bubbles around the important ports of Boulogne, Calais, and Dunkirk.
In the scenario I envision Hoffmann, who seemed to recognize the choke points in the British supply system, talks to Ludendorff who than makes Amiens the focus of Kaiserschlacht. That means focusing on the 18th army as the primary force tasked with bursting through the weak British 5th army, while the 2nd army to their north focuses on cutting off the Cambrai pocket from the flank and rear while fixing it with minor frontal attacks. It will also focus on fixing the attention of the British 3rd army north and directly South of the Somme river. It will progress no further than the Ancre river and seize Albert.
The 17th army is then not taking part in the offensive, as there is no reason to attack Arras and Albert from the North. Instead they just shell Arras and Bapaume, the chief supply line into the Cambrai salient, to confuse the British as to what their role is and to make the British 3rd army wary of dispatching reserves south. Obviously MARS isn't launched.
The German 7th army will participate with a large version of ERZENGEL to pin the French 6th army, which refused to use the elastic defense ordered by Petain due to the ideological views of Duchene, its commander, that will focus on clearing the north bank of the Aisne river and reclaiming the Chemin des Dames, while also seizing Soissons and Compiegne. This will have the goal of convincing the French that Paris is the main goal and to draw the French reserves in further South away from the German 18th army between the Oise and Somme.
Here too the Germans will make better use of their resources. Historically they only used about 42% of their heavy artillery batteries on the Western Front for MICHAEL, so here its bumped up to 60%. This is partly to aid ERZENGEL, but also to give the German 2nd army more guns. Also these batteries will allow for a greater transfer of forces north for GEORGETTE, which will be different ITTL. It will focus on taking Hazebrouck and bypassing Mt. Kemmel and cutting off the critical rail hub in that city.
Cutting both of those critical rain junctions should be enough to collapse the British supply system, which now would have only a single double tracked heavy duty line running along the Channel coast and through each port. It would back up supplies so badly that the British would have no choice but to withdraw or surrender to lack of food/munitions.
So quick summary: MICHAEL is the 2nd, 18th, and 7th armies attacking with the 2nd and 7th fixing the British 3rd army and French 6th army/reserves respectively. The 18th army will then have less opposition and can move more quickly to reach Amiens before the British can move up reserves, while also bypassing the French defenses of the city, built in 1915 and sited on the Ancre river.
Given that the Germans can hold Amiens when they take it, have enough troops to create a line to intercept the French to the South, while also making a British counterattack weaker and causing major supply difficulties, which become fatal when Hazebrouck is taken.
