You may have a point here although blocking merchant shipping by an allegedly neutral nation is definitely an hostile act.
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.
Not greatly larger than OTL while the Russians are acting on the defensive in their own homeland.
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.
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.