Without the USA, I think 1918 is too early for A-H collapse. German troops from the east will be used in France, sure, but without the prospect of vast US armies more will be diverted to the support of Germany's allies too. And if Bulgaria or the OE should collapse, they might be enough to form an army on the Danube or in the Balkan to keep Austria on board.
Right, but as I pointed out, American troops really weren't arriving in massive numbers at this point. It's not until September you really see a largescale American commitment (i.e., more than a couple of divisions), and even in terms of smallscale commitment it was fairly limited.
The problem elsewhere however is that by 1918, the Austro-Hungarian and Bulgarian armies had largely begun to disintegrate by themselves. In the former desertion was so bad that there were groups of men roaming the countryside with heavy weapons, whilst troops at the front were dealing with pitiful rations, and someone (the Kaiser) had the bright idea to relax discipline. In the latter a lot of the troops had no shoes (in the mountains, in Winter), and rations were even worse. Both these armies ended up collapsing not through battlefield action, so much as one hefty kick of the door bringing down the whole rotten structure. Vittorio-Veneto for example was a case of massive collapse, but one rather divorced from the battlefield circumstances.
The Ottomans had a different problem; simply being outpowered. Obviously one cannot assume things happen, but in real life by the end of 1918 (or even sooner if the Germans launch no Spring Offensive) the British (and minor allies) will be pushing into Anatolia and Thrace.
As for the German prospects of holding in the case of their allies collapsing, they're really not good. Even with extra troops available, the Germans would need to find troops to cover the Western Front, garrison the East, and cover a Southern Front extending from Switzerland to the Black Sea. Meanwhile the Allies will have so much extra strength to bring to the fight against Germany; the Italians up through Austria, the French, Greeks, Serbs and British from Macedonia, and the British and Imperial forces engaged in the Levant and Mesopotamia.
On top of that you have a British Empire which still has a lot of barrel left to scrape; the British never really utilised their colonial manpower reserves outside the Dominions (and even here South Africa and Australia got away without conscription, whilst in Canada it had no practical effect on manpower). Troops from Africa were not used in Europe, nor were Indians to a large extent. The French can also call on more men from the colonies, and indeed would be certain to do so.
The main problem you have if the US doesn't enter the war is not military, rather it is financial, and it is actually the most significant change and potentially a war-winner. By mid-1917 British credit would have run out, and the Americans had made it pretty clear that they wouldn't be so forthcoming with anymore.
Something akin to the Spring Offensive was in the cards regardless thanks to the troop surge from the Eastern front. Had they ever opened their eyes and realized the sorry state of the French... Then Kaisserreich becomes the default mode for Hearts of Iron
Edit: Actually I dont know about that last part.
The French really weren't in a sorry state. There certainly had been morale problems, but these were cases of military strikes against offensive action. The troops still wanted to win the war, they just didn't think costly and somewhat dubiously valuable offensives were the way to do that.
I do agree that there was almost certainly going to be an offensive of some sort in 1918 due to the troop surge. However, the French were in no sorry state by 1918. Even at their worst during the Mutinies of the year before, they had only opposed further offensive action; they were still willing to act on the defensive at least according to their own claims. Moreover, with most of their demands for better conditions fulfilled, their morale had recovered sufficiently by 1918 to permit the resumption of actual offensive actions, much less further defensive action against German attacks. While the major question might be how much it was reliant on the morale boost of the Americans, we know that historically the French were capable of taking on most of the attacks of the Spring Offensive directed at them rather than at the British: Matz and Second Marne, to wit. Even where the German attack enjoyed success due to incompetent commanders at the Third Aisne, the offensive petered out due to logistical matters before it could take Paris (again), and the counterattack again knocked them back. Ultimately, the French Army of 1918 was nowhere near the condition of, say, the Austro-Hungarian Army of late 1918 where national troops refused to act in defense of the whole.
In fact, that actually ties into what I believe would be most likely, which is that the war would be won elsewhere. The Vardar Offensive in the Balkans saw no Americans and had the dual effects of knocking the Bulgarians out of the war and opening up Hungary to direct attack. With the Allies across the Danube by November, the Hungarian government sought a cease-fire that ended the war on the Italian front. With the Austrians also seeking terms due to the secession of most of the entities within the nation, this opened all of southern Germany to the threat of Allied action slowed only by the speed that they could actually get there. This came as a complete and unwelcome surprise to the Germans, naturally, who had believed that their allies, even Austria-Hungary, could withstand more such attacks. With Italians in Innsbruck and the Greeks and Serbs moving north as fast as they could march, the Germans were not looking at an auspicious strategic situation regardless of the position of the lines in the West. This is essentially in accord with DoomBunny's post outlined earlier, just in a few more words. ^_^
Salonika is very interesting as a front for this reason really. A French brainchild from the start, it's somewhat of a mixed bag.
Initially it was nothing more than a massive waste of strength; essentially dumping troops into a huge POW camp and giving them a hefty dose of malaria. But, once you reach 1918 it really comes into its own as a front. The Macedonian offensive collapsed the Central Powers in the Balkans, knocked out Bulgaria, dealt the Germans a hefty morale blow, opened up the way into the Danubian plain, and in an effect often forgotten was actually responsible for Ottoman surrender as well (cut off from all allied aid and with Allied troops marching on Constantinople had more effect than what were admittedly brilliant campaigns in the Levant and Mesopotamia).
Whether it was worth it in the long run is therefore somewhat debatable. The pay off was great, but the few years beforehand were troops sat around getting malaria and counting blades of grass less so. Then again there really wasn't anywhere else those troops could have gone other than the Western Front, so.