Good questions Ruaidhri, but try not to look at whether Mugabe himself was long term or not (& clearly 27 years is a very long time for one man to rule any country); rather look at HOW he ruled and the WAYS in which Zimbabwe tried to muster legitimacy for its regime. Read the attached link from the BBC. You will see it littered with attepmts to legitimise the regime – parliament; party votes; constitutional rules etc.
In terms of NI, it is true to say that we have just had our first full parliament since the collapse of 1972. The legitimacy that was lacking from 1999-2002 was that one of the main parties (DUP) opposed and tried to ‘wreck’ the institutions from within. Since 2006 (St Andrews Agreement) all democratic poltical parties have recognised the ‘rightfulness’ or legitimacy of NI’s democratic institutions. There are continuing issues with things such as voting procedures (e.g. parallel consent) but all the main parties accept the institutions in principle.