You may have a point here although blocking merchant shipping by an allegedly neutral nation is definitely an hostile act.
The British blockade impeding Dutch, Norwegian, Swedish and American shipping to Germany from 1914-1917 says hi. Again, what you consider hostile was not necessarily considered hostile back then; there was far more legal room on this issue back then.
Possibly although one alternative could be sending forces to the Balkans via Montenegro, which unlike Serbia did have a coastline, although not sure how capable the logistics were.
Non existent and ignores that in 1915 the best they could do was send a pair of divisions to Greece.
Not greatly larger than OTL while the Russians are acting on the defensive in their own homeland.
Vastly larger, given the Italian Front had 61 Austro-Hungarian Divisions in 1915; you've effectively doubled the Central Powers force ratio with the Russians. As for the Russians being on the defensive, that didn't work out too well for them historically, no?
No it hasn't. Russia has spent one campaign waging a fairly successful offensive against the Austrian empire - along with the disaster in the north. Now its defending its homeland, especially after a withdrawal from the Polish salient, which will lengthen CP lines considerable, rather than launching very costly attacks against enemy positions. If the Russians had 1917 army/positions with 1915 morale then the Germans will struggle a lot more and the Austrians are likely to suffer a lot worse. - True the Russian army won't have the same level of equipment it had in 1917 but neither will the CPs.
You seem to be convinced the Russians suddenly woke up in 1917 with low morale, instead of realizing said morale was a ramification of having lost the same territories and taken the same casualties you are conceding they will be suffering here. Here they're going to lose the same territory and taken even more casualties, in a shorter time span, so why exactly are we assuming they don't have 1917 morale to match a 1917 style situation? Even if they don't, the Russians will weaker relative to the Central Powers in both material and manpower constraints, which opens up greater operational possibilities against them.
What was used against Turkey - including operations in Persia and probably reserves from the Far East. As well as new recruits being entered into the army as it still has a lot of people to call upon in 1915.
Oh I full assume the British will still be mobilized to the same extent as historical, so that's no change. If we are assuming those "reserves" are detached from their postings, you need to stop and consider why they were there in the first place.