I agree that the Principle of the Identity of Indiscernibles (PII) doesn't seem, initially, to "do any work" in Leibniz's argument. I've been trying to get my head around what L. means and am beginning to understand what he is saying (I think). At first I thought PII was tautologous but on reflection my view has changed.
L. is using PII to argue that two indiscernible things could only be 'separated' by space and time, hence if there were indiscernible things separated by space and time then the properties of space and time become properties of the objects. L. doesn't want this argument to succeed, therefore he argues that there are no indiscernible things, in terms of all of their non space time properties.
I'm not sure I agree that L. doesn't use the Principle of Sufficient Reason. He is arguing that were the universe to be created earlier or in a different place, there would need to be a reason, and there couldn't be such a reason, therefore it couldn't happen? Again, if the universe were to be created in a different space or at a different time then space an time become properties of the objects and therefore have some absolute existence and are not merely relational.
Both arguments certainly stretch the thinking, but I was wrong to rush to judgement.
I do think however, that L.'s philosophy is driven by his Theology. I also agree with him that there probably aren't two indiscernible things (excluding their position in space and time) at the macro level but I'm not sure this achieves what L. wants.
It could be possible that space and time are both absolute and relative. I'm not sure whether it is fair to argue that these two views are mutually exclusive.