The unsecured loans after April, 1917 were a combination of the US government guaranteeing them (because the USA was in the war), and some hope that the money could be squeezed out of the defeated CP. If the USA does not enter the war in April, 1917, then the Entente won't be getting unsecured loans from private financiers in the USA as there will be no US government guarantee. This means that the only way the Entente powers can buy from the USA is with cash as their collateral (at least that in the USA) is used up. Now a private business supplying the Entente, for example a factory making uniforms, might sell on credit (with interest) but this is going to be limited as this factory needs cash to purchase raw materials, pay workers and so forth. At some point, especially if repayment is slow and the Entente keeps mumbling "refinance at higher rates" this will dry up, and in any case will never approach the level in aggregate of the major loans being made. To some extent collateral outside the USA might be accepted, but I expect limited in amount and at a significant discount as you can't count actually getting your hands on it, especially fixed assets.
As far as the Brest-Litovsk "bonus" for material, yes this is limited in 1918 but will be better in 1919. The ceded area has significant food exporting history, as well as coal and other needs. The Germans of 1918 are not the Nazis of 1941. The peoples of the ceded areas may (or may not) be pleased with the change of government, certainly some will find the Germans better than the Russians - the Balts, the Jews, the Poles will find life somewhat looser.
Without the 1 million Americans that came to Europe between April, 1917 and November, 1918 in not just combat units but support units that helped rebuild and expand French railways, upgraded ports etc all of those tasks - manning every inch of trench, repairing every miles of track, and much more needs to be done by Entente manpower. This means manpower for the Balkans, manpower for the home front (fewer men need be called to the colors) is much tighter for the Entente. It is worth noting the plans were for by Spring, 1919 for 2 million Americans to be in France. You also don't have the USN helping out in the Atlantic.
In this scenario, the Italian front is certainly not going to do A-H in and any Entente efforts in the Balkans are going to be reduced due to the demands of the Western Front absent American participation. By fall, 1917 Russia is out of the picture so A-H is looking at a defensible front in Italy, and a front in the Balkans with terrain in their favor and reduced Entente pressure. Since their situation is not as dire, yes they are shambolic and a weak reed but lasting longer is quite reasonable especially basically on the defensive.
The odds are neither side is going to be able to "win" on the decisive western front. If the Germans rationalize their lines and are in the sorts of fortifications they either made or planned (Hindenburg line complete) they can conserve resources and bleeds the Entente. Tanks are not where they need to be to be decisive for a serious breakthrough. The issue is the politics in Britain, how long before there is a push to get a fair peace - and one that restores most of the prewar lines in the west would be acceptable to the UK. The French are already bled white, and while the mutinies of 1917 DID NOT mean the French Army would not fight like tigers on defense, massive offensives with equally massive slaughter led by "les gros légumes" would, IMHO, rekindle mutiny on a much grander scale. The net result is Germany can most likely manage to keep on on the defensive, the British and French for a variety of reasons can't win and are exhausted. Sooner rather than later a peace not quite status quo antebellum is arrived at in 1919 or 1920 at the latest.