Jerry,
Turkey may have felt that the 1974 invasion was legal but displacing one third of the population and replacing them with settlers was surely reckless and above the law since the Treaty of Guarentee did not permit this.
You're right if your trying to separate the legal arguments about the 1974 intervention and then what happened afterwards, especially the further from '74 one moves. Justification for the intervention
does not give justification for (a) expulsion or (b) settlement. It seems to be possible to justify the intervention without condoning the expulsion or settlement. In fact a strong argument could be made that until the policy of bringing in settlers - was that 1977, later, earlier ? - Turkey had a a reasonably strong legal position. The moment it effectively failed to follow occupation law i.e, don't change the pre-existing laws and institutions, then the its legal position got weaker and weaker. It may be, and I am just speculating here, that the realisation that its legal position was getting weaker gave impetus to declaring the TRNC.
Yes, the T. of Guarantee makes no provision for settlers, but then again one wouldn't expect it to. The T of Guarantee is a limited instrument, it is not an open cheque, and again going back to an earlier posting, any ambiguous elements of the T of Guarantee are hedged off and delineated by other instruments of international law. Thus though the T of Guarantee does mention settlers and settlement, a number of the laws of war and occupation do so these other provisions then kick in to prohibit permanent expulsion or settlement.
Does the principle that agreements signed under duress are invalid extend to international constitutional agreements/treaties?
This is a Catch-22. One could point to thousands of agreements which were negotiated or signed under duress or at least under 'offer you can't refuse' circumstances. (One could start with the demand for unconditional surrender at the end of WW2) At the moment of signature/ratification/entry into force few would admit it was done under duress. But unless it is made clear that it was under duress at the time, then there is absolutely no chance that it will be nullified. (Again, look at the 1969 Vienna Convention at the law on this). The assumption is that adoption of all international legal instruments by free and equal sovereign states - whether a Malta or a China - are freely entered into. Basically it is hard to show - as the leaders smile for the photographers at the signing ceremony - that any agreement was the result of duress. The best we've got are the political memoirs decades after the event to tell us otherwise.
You might be labled a "Turk Lover" if you were soften a bit...... 
