United States of Greater Austria

I feel it would have at least delayed war, if only by assuaging nationalist concerns.

  It won't solve all problems but it would help immensely.  Whether it could be implemented fully in terms of probable opposition by invested interests, especially the German and Magyar aristocracies is a serious issue.  There were however also significant economic, historical and personal connections tying the state together, which helped it last as long as it did. 

  Ultimately there is the problem in a period where national identities are very powerful of can the state survive?  However Switzerland managed it, as so far, has Belgium and there are a number of other states which have multiple nationalities.  As long as the grip of the current ruling elite is loosened it should have some chances to develop into a more successful nation.

What about if Britain will gradually switch over to Germany's side after 1916-1917 due to Russia's growing military power, though?

  That might have occurred, especially if Russia started looking more threatening and Germany became more diplomatic.  An end to the naval challenge and acceptance that other powers interests have merit would have helped greatly.  Also if a stronger Russia not only looked likely to upset the balance of power in Europe but seemed a greater threat in areas of Asia where they clashed with British interests.  [I.e. China, the straits & Persia/Afghanistan].  I suspect it wouldn't really shift dramatically until say 1920 because there would be a large level of inertia in such circumstances.  Unless possibly a Franco-Russian war with Germany looked like winning an overwhelming victory.

  However apart from anything else:
a)  The basic idea is that the Haspburg empire sees fairly dramatic reform.  If so by about 1920 it would be likely to be a useful defensive ally for Germany against an aggressive Russia, which would balance things a lot.

b)  As WWI showed attacking against prepared defences is difficult and often costly and Russia's military strength was over-rated given the actual performance of the Russian army in the offensive and its social and political weaknesses.

 1)  True but the Hapsburg empire stood up reasonably well to pressure OTL and with say 5-6 years of reform and standing on the defensive with a more motivated army it could cause a 1920 Imperial Russian army some problems at least.  Both powers are likely to still be of fairly poor quality but the Austrians are likely to have an edge.

 2)  True but without WWI and the experience of the problems of breaking through defensive lines how rapidly would either armour or air power be developed?  Especially if we're mainly talking about a not particularly reformed imperial Russia that is likely to reply on large numbers to counter its poor quality [in terms of poorly trained,  equipped and led masses are a lack recognition of the need for well trained and technically educated officers.

  To clarify I suspect that Britain might well get drawn into such a conflict, especially if it looked like a Franco-Russian bloc was the aggressor, was winning and seemed intent on fairly draconican terms that would upset the balance of power.  However I think it would probably stay out of the early stages because of the lingering distrust of imperial Germany, plus possibly being distracted by internal matters.   [I.e. Ireland, social and education reform, tariff policy etc.]