One of the critical points in this election for unionists was in March 2013. That was when Martin McGuinness stood down as MP and in the ensuing election Francie Molloy was elected. Nigel Lutton (whose father Frederick had been murdered by the IRA and Molloy connected with that murder in the House of Commons by the DUP’s David Simpson) stood as a united unionist candidate. That resulted in a modest increase in the unionist vote which Mike Nesbitt claimed as justification for the decision (also possibly beneficially for the UUP’s internal cohesion it led to the resignation of Basil McCrea and John McCallister).
A number of things became clear after that:
Nesbitt supported the concept and the majority of his party would cooperate.
The other unionist parties would also cooperate and support a unity candidate.
The unionist electorate (at least in Mid Ulster) supported the concept
That modest increase in unionist vote could be argued to be the basis of the significant gain for unionism at this Westminster election. It needs to be remembered that unionists have lost seats at most Westminster elections for years (the last gain was Willie Thompson taking the new seat of West Tyrone in 1997 on an evenly split nationalist vote).
The results bear individual more detailed analysis wich I will try to do later but in sum: unionism gained two seats, made a third safe and would, very likely have won another had there been a pact in South Belfast.