Pedantic point:
Your cousin didn't obtain 'probate' (as that only applies when there's a will). He obtained a 'grant of representation' (in the form of 'Letters of Administration') to distribute the estate in accordance with the intestacy rules (rather than the probate ones).
As Administrator, he was
personally liable for ensuring that your uncle's estate was distributed in accordance with the intestacy rules. They're explained here but, from what you've previously written, it appears that the whole of the estate should have passed to your mother (or, as it turned out, to her estate):
https://www.gov.uk/inherits-someone-dies-without-will
My use of 'personally liable', above, means that if got the distribution of your uncle's estate wrong (whether wilfully or simply through his ignorance of the rules) you, as the executor of your mother's estate, can sue HIM for every penny that should have passed to that estate. (It's irrelevant as to whether he's able to recover the money from those who erroneously benefited. HE got it wrong and HE must put matters right).
Time to consult a solicitor, methinks!