Validity of Special Power of Attorney (SPA) after death of Executor and enforceability of Civil Suit

In 2010, X has executed a Special Power of Attorney (SPA) [unregistered & notarized] in favour of Y to look after two Court Cases in a property dispute matter viz. civil injunction suit and FIR. The said SPA was never brought to the knowledge of the Court during the lifetime of X, who expired in 2012. In 2013, one Y approached the Court stating that I am SPA holder of deceased X. The Court allowed him to reinstate the case and in 2022 / 2023 passed an ex-parte decree and treated Y as LR of X. The LRs of the deceased neither comes into picture nor Y claims himself as the "class-1" or "class-2" legal heir during the entire civil proceedings. We challenged the decree in First Appeal. We have questioned the validity of suit on the basis of invalid SPA after death of the executor, forgery of documents and many other grounds. Moreover, Y suppressed the material facts of decision pronounced by criminal court by acquittal of all the accused persons, while treating X and ourselves as the parties who were cheated by vested interests. Any guidance or citation on the subject matter. The appeal is at final arguments stage.