In an alternate Russo-Turkish War, is there a possibility that the sides evolve into something resembling this:
Russia, Austria-Hungary, Germany, and Italy (including minor powers)
VERSUS
Ottomans, France, Spain, and the United Kingdom (including minor powers)
If it did somehow evolve this way, what would the war look like, who would be the victors, and what would the peace treaty(s) look like? Thank you for your time!
Not exactly that combination but ....
In the mid 1870's there Germany was considering a new war with France. Russian neutrality was a prerequisite. As a reciprocity Bismark presumably was ready to support future Russian-Ottoman war and resulting Russian control over the Straits. In OTL Alexander II personally interfered on French behalf. When the anti-Ottoman uprisings started in 1875 - 76, diplomacy had been doing rather pointless diplomatic dances for 2 years before declaring war on the Ottomans.
To get what you want, in 1875 Alexander II travels to Berlin to make a deal: Germany has free hands regarding France while Russia is free to attack the Ottomans. Issue of the Straits is agreed upon. AH would have no option but to join and to get its reward, Bosnia and Herzegovina (in 1875 both are in rebellion against the Ottomans).
1876 Serbia and Montenegro declared war on the Ottoman Empire and Bulgaria rebels. Russia declares war on the Ottomans. AH either joins or is friendly neutral (for a while). Germany attacks France.
The Brits at that time don't have too much to contribute on land (in OTL they were prepared to send a single army corps to Turkey). If they can land some substantial force in France (which I doubt), then Germans could be somewhat slowed but not stopped. Not sure why the Brits would get themselves directly involved into such a mess.
France is going to be defeated by Germany (I have no idea how long would it take at that time) and, without the OTL "diplomatic interferences" by Tsar, Russian war against the Ottomans is going according to the initial Obruchev's plan which means that in 3 months their army is near Istanbul.
AH would be forced by Germany to get on board, which makes Ottoman situation hopeless (I wonder what may change if Italy declares war on AH).
A question: Serbs would like to get a full independence from the Ottomans but would they be happy if their liberators are Austrians and not Russians?
With the Straits controlled by the Russians, British appearance in the Black Sea is impossible. Their appearance in the Straits area is pretty much useless. On the Baltic Sea coastal fortifications of Sveaborg had been brought up to date so the French-British navies can't do any serious damage. Not sure about status of the German and Russian Baltic fleets at that time and their ability not to let the enemy to get in. Anyway, at best, the allied squadrons can do some shelling of the German and Russian coast. After capitulation of France and the Ottomans the British can't continue war in any meaningful way.