By only looking at the sustainability of the Greek governmental debt you only solve a part of the problem. In the Debt Sustainability Analysis (DSA) of the IMF GDP is used as an exogenous parameter. This analysis shows a very vulnerable situation with respect to all parameters, meaning that it is very unlike that the proposals presented so far will solve the Greek problem.
The austerity program where so many Greeks are protesting against is unfortunately the only way to go. The main two valid arguments for that is the fact that the productivity is approximately 20 % lower then in The Netherlands which should result in a comparable lower GDP and wages and secondly that they are locked to the Euro. The good thing is that in the last 5 years this is already achieved, though in the hard way. So now the Greek can compete on the European market again, reducing their Current Account Balance (CAB), necessary because the deficit has to be turned into a surplus to make debt sustainable.
But here is the problem, you cannot require both the deficit and the CAB to be at a certain desired value because it will result in a not sustainable real economy, with high unemployment and with low probability to recover GDP, resulting in long term secular stagnation.
Apart from necessary tax reform, reduction of corruption and lowering prices of medicine among others, there is a need for a long term lower interest rate in combination with a at least 25 % haircut on debt to put Greece on a really sustainable track. This means a write off for all the lenders to Greece, Banks, countries, IMF, ECB, Central bank, etc. First responsibility for equity holders than deposit holders and as soon as the ERM is in place the European Resolution Fund and if needed the Governments have to support/bailout banks which are in trouble, because who else can save a institution relevant to society from collapsing, right: the taxpayer, but as a last resort saver.
For the Greeks the most important task is to reduce unemployment and stop the exodus of high educated young people who simply want to be part of a reasonable society, but reducing unemployment seems to be a problem in the whole European Union.
Well, still one day to go for the final(?) decision of the Euro group concerning the GREXIT.