Probably fairly sound ones
I should note I turned up what proportion (admittedly in terms of value, other measures are also valid) of British imports were from the USA and the figure for 1918 was 39.2%
Per Kathleen Burk, Britain, America and the Sinews of War 1914-18
Now it should be noted that the value figure hides the exact nature of supplies and some items are of course more of a bottleneck than others but...since the value of British exports to the US covered of the 15.9% US imports in 1917 and 5% in 1918 you are looking at perhaps a 37.2% reduction overall.
Now admittedly some items (explosives, oil) came rather more overwhelmingly from the US than any other source so you are probably right not to assume 37.24% reduction the Brits will be 62.5% fine. However you may begin to see why I have been so sceptical of total Entente collapse.
While it would be hard to guess at particular terms likely a German retreat to something more like its 1914 borders and certainly from Belgium would be on the cards. Items like seizing the High Seas Fleet given the Entente would be hurting themselves at this point would require extremely ballsy negotiators and I would not tend to anticipate such. An awful lot would depend on the exact state of the German field armies, 1918 saw them simply collapse which of course magnified the demands the Allies (Entente plus USA) could make, here the Germans are not without a reasonable chance of a somewhat strong bargaining position of their own. There may even be territorial adjustments in Germany's favour in the east.
However given A-H was going down and sooner rather than later and given that to beat rather than simply hold off the Entente the Germans would need to undertake a successful offensive into France while the Entente at this stage just need to hang on I don't see Germany winning a major victory as the most likely or even the second most likely outcome. I do see Germany likely getting terms much closer to something out of one of the 18th Century Wars which often came to a close because of mutual economic exhaustion however.