can they even interpret it to effectively negate the rule of not using domestic law as a justification for non fullfilment of obligations by interpreting the treaty in a way that suits them ?
There is a lot behind the word "can" in your question. If you are simply asking whether states have the sovereign power to interpret international law however they want, then sure. But they might be wrong (thereby leaving themselves in violation of the treaty they are failing to implement), and if they are intentionally doing so to avoid fulfilment of international obligations, then this is itself a violation of international law. States must take actions in good faith towards implementing treaties in their domestic law. See Articles 26 and 27 of the Vienna Convention.
A nation can interpret its treaty obligations however it wants. If it is wrong in its interpretation, the nation might be sued which could end up being somehow penalized for a wrongful action based on an incorrect interpretation. Whether they can get away with it depends on who is going to enforce that law.