I would set this up as follows: the deceased noble had a liasion with a servant or someone of low class who now stands to inherit his fortune via a bastard son, if no-one else can be found to claim the inheritance. This would scandalise the upper echelons and so a particular conservative judge or other noble is determined that no-matter what, this low class scum will not be elevated to the nobility.
The con-artist and his crew hear of this plump goose of an opportunity and breeze in from out of town with false papers etc to try and claim the inheritance. They will try and set up a long-con, but the target is this judge, who already has a vested interest in the public hearing finding in favour of the "noble outlander"; i.e. the con-artists.
There is to be a public hearing on this matter, where any with a claim can state it and have it investigated. The preliminary hearing has two serious claims: the low born and the con-artist.
However, the public investigator assigned to look into this matter by the judge has an old grudge against him and tries to thwart him once he understands that the judge is trying to influence the outcome.
The PCs are "hired" by the low born girl and her son on the understanding that they will get something out of the inhertance if they win.
Now you have a nice game of complex cat and mouse. The con-artists are trying to convince the judge, who is lapping it up, and not paying much attention to what he is being shown, but the public investigator is looking carefully, and is trying to find any evidence to support the girl, to enrage his old enemy.
The judge's attention is on fixing the jury for the public hearing, so that the public hearing will find in favour of the blue-blood, i.e. the con-artists (the alternative in unthinkable to him). He will also hire villians to try and discredit the girl and pay people to say her claim is false etc, or try to pay her off etc etc.
The con-artists will also be trying to convince the public prosecutor, the judge and the jury of the public hearing that they are genuine and will be producing witnesses (paid), relatives (disguised associates) etc to try and bolster their story.
If you want to make things really complicated, remember that the con-artists do not exist in a vaccuum. Perhaps one of the con-artists has a lover that the PCs can try and get to talk. Then assassins can be hired to fix this problem once she has blabbed a titbit but this is useless unless she appears at the hearing...........cue the assassins................
Also, how about if one of the con-artists is recognised by a past associate and blackmailed that unless he pays, the blackmailer reveals all to the authorities. Maybe the PCs hear about the blackmailer by intercepting cryptic notes to the con-artist. They don't spell things out but are intriguing enough for further investigation.
Maybe a relative realises that the person the con-artist is pretending to be is impossible and confronts him, where-upon he has to kidnap the offending relative until after the con is over: cue safe house and also a replacement, who might be magically disguised/doppleganger etc. Or else perhaps they dominate the relative and make them tell lies and the PCs have to work it out from the fact that friends of this relative think he/she is acting strangely.
I hope this helps. Not sure if I am on the right track but it should get you started.
