Peace? Very probably...Germany getting what you say? Not really.
First Italy as it's more or less my 'field'; sure if the Austrian know that a relief force is coming maybe (and i say maybe due to their supply situation) they will not break totally, but remain the fact that they will lose badly and frankly the relief force is not a give; basically once the offensive end and the italian form a defensive line...a breakthrough will be neither easy or cheap and this will be a very important factor for the Germans as Italy for them is an irrilevant front by now and is cheaper give them some bit of Austria-Hungary than continue the war
All the continued fight in France, the destruction of the BEF, etc. etc. they don't come cheap in term of men and material and by 1918 Germany had not much more to spare either and regarding France, sure in 1917 there were munity...but the problem was that the soldiers refused to continue new offensive not to stop defending the country.
Not counting that the rest of the CP allies are not in a very good shape...and i'm being optimistic here.
Sure Germany want their colonies back, BL recognized, Belgium etc. etc. etc....and for news i want to become emperor of the world, and by 1918 Germany had the same change of me to achieve what he want. Sure France is out, but also Germany is spent, the blockade still on...with famine running rampant in the motherland, Poland and Ukraine on the verge of rebellion, Austria-Hungary not looking very well and seem in need to other troops and supply to finally defeat the italians; basically they are in the same situation that you describe France.
The UK will continue the war, because by now has invested too much in the war and frankly with the loss of the BEF the only way to hurt Germany is by blockade (that don't cost that much...relatevely speanking) and giving a little support to Italy
In poor words, by this stage they can get a lot...but not everything they want and as i said they will need to decide what to toss away and what to keep.
The British giving back the colonies to Germany is difficult, even because many are in the end of the Dominions and this mean London forcing them to giving back; there is Congo and as i said Berlin can decide to have Belgium or get Congo not both. Hell, is much more probable that London will seize the german colonies in French possession if the peace is too harsh...and there is nothing that Berlin can do to stop them.
The plausibility of the initial scenario isn't even the topic here, but the plausibility of USA declaring war on both Japan and UK after Pearl Harbor, given that Anglo-Japanese alliance stands strong and American-British relations aren't too good. But let's go anyways:
The players of the Great War in start of 1918:
Entente:
-Russia: rekt
-Italy: has lost way too many troops, though army is adapting. This far has lost bitterly, though OTL managed to defend Piave and mount a large attack to crack Austrian lines later on. Marginally weaker than OTL but really
-France: has suffered heavy casualties but is still standing, OTL homefront was looking pretty bad before US aid came in, with that out France is running out of money, food and morale. Civilian protests occurred IRL both in 1917 and around Spring Offensive. Now it will all be one continuous time of troubles. Home front is something that lost Russia and Central Powers the war IRL, though CP defeat would have come anyway. Point is, it is not to be underestimated.
-Britain: has lost much more than they wanted to but relatively intact by comparison, practically unlimited manpower, but OTL was depending on US supplies and loans for the final year of the war. Here they have much less those -> weaker home front, though not to the same extent as French home front
-Belgium: On British life support, basically not much more than an army
-Serbia: same as Belgium
-Greece: Doing pretty well, though can't harm any of the main three Central Powers. OTL helped to crack Bulgaria later on
Central Powers:
-Ottomans: rekt almost to the same extent as Russia, though still pinning down some Entente troops. Empire might or might not be salvageable after the war, but they won't contribute to the victory at this point. They have no way of helping Western Front.
-Austria-Hungary: incompetent and suffered much, like Italy and Russia, but so far still standing. Homefront vulnerable, but all they have to do is to not lose before the French.
-Germany: the blockade hurts them, but morale is suddenly high and there is hope of victory with Russia falling. Homefront maybe not quite as stable as Britain's but still one of the best here. Didn't surrender OTL until all of their allies had crumbled down
-Bulgaria: doing okay-ish, though morale low and economy in shambles. Military-wise there's a stalemate which looks stable for now
1918 will be simply a German storm. Spring Offensive starts as OTL but lower Entente morale leads to Germans gaining a lot of more ground. With the front line close and a lack of manpower, French military command becomes increasingly concerned, and counter offensives fail to push the Germans back decisively. French army does not like getting thrown at German guns for little gain, just like in 1917. Fourth operation of Spring Offensive brings Germans quite near Paris, and civilian panic ensues. Even OTL there were talks about leaving Paris. Second last part brings French troops concentrated around Verdun (which is now a French salient) and Paris, while the last push isolates and finally breaks both BEF and Belgian Army in August. If even this isn't enough, now Germany has nearly free hands in France since they outnumber the French and have now removed their allies. Germans probably COULD take Paris, head-on, though it would be extremely costly. However, simply achieving breakthrough is enough for victory, and as French numbers and morale fall, it becomes a real possibility. As a third option, which is maybe the safest, is continuing to push from both sides of Paris and just have a huge siege for the old times sake. They will run out of food and then France is completely done. Bonus points if they can't leave the capital before they get surrounded. However I firmly believe that the surrender of BEF is more than enough for France to just quit. Would Britain continue after France is out? Just maybe, but not after both France and Italy are out. If Austria holds longer than France, Italy can't do shit once the Germans get there in large numbers. If Austria wasn't completely broken, and Italy still had will to fight, Germans will sooner or later break the line and force a peace.
So can the war be won before France surrenders? Both Italian and Macedonian fronts look promising, but Macedonian front can't achieve anything major and maybe won't even be won if France and UK pull back once France is getting fucked hard. If they win, it's still just Bulgaria. (though OTL it was the final straw that prompted Ottoman surrender) However, if Italian Front AND Macedonian fronts are won, there is still a (small) chance for Centrals to break and UK to stay in war. Thing is, the timeline starts with Centrals winning, so I can't allow that to happen. If necessary for the timeline to work out, I'll just make either Piave work or Vittorio Veneto not work with magical butterflies. For instance, the success of Germany in the West and the faltering French morale pressure Italy to attack too early, and it goes about as well as Piave did for Austrians. Even if they fuck up completely and Italians are running towards Vienna, will they surrender before the French? It would certainly be interesting to see Belgium, France, Austria-Hungary and Bulgaria all fall around the same time, leaving only Ottomans and Germans versus Brits and Greeks, but I believe that Britain would fight on in that case.
So basically the options here are:
A: My original plan, Germans focus on naval action with British Navy trying to harm the blockade, which doesn't need to do anything but to keep USA out. Spring Offensive starts as OTL, weaker French and British morale ends up with France surrendering after BEF breaks. Italian Front doesn't matter, it's just too late and total breakthrough never occurs, just retreat. Italy surrenders, knowing it is fucked, as does Greece. Britain sues for peace.
B: Same as above, but France holds on a bit more. Italian Front becomes relevant in a race to break one major nation, but disaster is averted, Austria defends, though barely, and Germany takes Paris or breaks the French totally in winter 1918.
Britain might send some help to Italy before France falls, after Italy is broken they again sue for peace. Now there is enough time for Bulgaria to break, probably, but Ottomans won't surrender as they still believe in Germans.
C: As original, but Italy attacks way too early because of pressure in France. A disaster happens, prompting the Austrians for a final attack, this time extremely successful. Basically reverse Piave and Vittorio. Italy breaks before France, A-H survives and actually sends help to other fronts. French surrender with Italy and BEF falling, Austrians save Bulgaria. Britain definitely sues for peace.
D: Piave works out for the Austrians, even earlier Austrian victory. Italian rout and a nice breakthrough for Austrians who finally feel like they aren't completely useless. Austrians save Bulgaria again, France goes down soon enough and Britain sues for peace.
E: Piave works out, but France really doesn't want to surrender. British try to land a second force in Bretagne, but it is too late and too small. Continued German attacks surround Paris and Verdun, and threatening Vosges and the coastline, and French army is broken whilst trying to relieve Paris. Germans are really running low on manpower but Austria attacks through Italian territory (something Italy agreed to) and sends troops to the eastern pincer around Paris. Germans clash with small parts of the French army and the new British force for a few weeks until finally a white flag is raised at Paris.
While there are many ways of making Entente win without America, they do not apply here. Entente lost, that's the point. Now what happens to Japan, USA and Britain?