IV, if I may give my reasoned opinion, it is actually rather unlikely that Britain shall be an enemy of the Habsburg alliance in the first general European war in the 50s-70s. Russia and France shall be the enemies that round, with Turkey possibly joining the Habsburg. Yep, the Schleswig-Holstein question is sgnificant and might theoretically be a casus belli, but it was not very important to London, it is much more likely that Britain mediates a compromise after Germany humbles Denmark, and afterwards Scandinavia turns inward to attend to its own unification. In this period, Russia and France were the main imperialistic rivals of Britain, the UK was friendly to German and Italian unifications, at this point the Habsburg block is a land power which does not directly threaten any UK interest, and no more destabilizing to European equilibrium than the Triple Alliance, which Britain was happy with for 20 years. Britain is unlikely to make a 180° turn in its foreign policy for little reason.
Britain is much more likely to join the *Entente in the second round, in the 1890s-1910s, when the Habsburg bloc has started to be a major player in the colonial scramble, has in all likelihood already humbled France and Russia, is economically upstaging Britain and possibly entered a naval race with it, and thus became a believable threat to UK interests.
To sum it up:
*Schelswig-Holstein war: Germany vs. Denmark, 1848-49, German victory, compromise peace mediated by Britain (Hostein to Germany, Schleswig to Denmark or partitioned.
*Crimean/Franco-Habsburg War: 1855/1870, Germany, Italy, Hungary, Galicia, (Turkey) vs. France and Russia. Britain true neutral.
*WWI: 1890/1910: Germany, Italy, Hungary, Poland, UBD, Turkey vs. Britain, France, Russia, (Serbia, Romania, Greece). America true neutral.