Thinking of the Manchester situation and of Trollope's novels (my best friend's husband was a Trollope scholar), I wonder if
a) any other heir apparent to a peerage, believed to be legitimate, was later found to be illegitimate because one or the other of his parents were already married (and the previous marriage not dissolved properly).
I can't think of any cases here. The 7th and 8th Earls Fitzwilliam had some issues with relatives claiming that the 7th Earl Fitzwilliam was not the biological child of his (married) parents, but that is a whole different story. I think some Cavendishes felt the same way about "Hart" or the 6th Duke - believing that he was the child of the 5th Duke by his mistress.
Fortunately, the Earl of Euston (who married unsuitably) had no children, because he tried to divorce his wife on the grounds that she was already previously married at the time. She countersued and proved that her first husband was himself married at the time, so she was actually not legally married. (Thanks to the late William Addams Reitwiesner for pointing this out to me nearly 18 years back).
b) any holder of a peerage was found to be illegitimate - either because his parents were not properly married, or because his father could not have been his biological child
In the Russell case, the late 4th Baron Ampthill was declared legitimate by the courts, which means that even genetic evidence would be discounted - if the House of Lords accepted the court judgement to be superior to their own. The late Lord Ampthill refused to take any tests, although his half-brother would have liked one. (This was in the 1970s). I assume that even in the future, if it is discovered that Geoffrey Russell was not the biological son of the 3rd Baron, the peerage would continue in the line of his second son and his son (his elder son the 5th Baron having had no daughters).
c) any claimaint to a dormant or extinct peerage was found to be descended from an illegitimate son/daughter -
I'm thinking of the Knollys cases, where the presumed son of the Earl of Banbury was refused by the House of Lords as was a descendant circa 1800, since their mother was openly living with another peer. (So much for English law presuming that the mother's husband was the father if he lived in the British Isles. Not if the title claimed was a peerage...) Their descendant became Viscount Knollys.
There was a wonderful story Man and Wife by Trollope about the peculiarities of Scottish law and how it was used by one woman to free another woman from a dreadful marriage.... A future earl of Stair found himself with two wives briefly - one by Scottish law and one (Lady Laura Tollemache? or Manners?) by English law. That ultimately freed the English wife (as a victim of bigamy), but she could never remarry. And the new earl divorced his first wife. Fortunately, there were no children. As for the 2nd Earl Russell, brother of Bertrand Russell, who also found himself a bigamist and was actually imprisoned for that, unlike Mr Dalrymple (later Lord Stair).
Shinjinee