About a possible Neapolitan match for any of the Borgias, why not marry Sancia to Juan/Giovanni instead of Gioffre? After all, she had no problem having an affair with him and Cesare after marrying a younger husband.
Then, secondly, if Carlotta (the later comtesse de Laval) refuses, why not marry Cesare to her half-aunt, Giovanna (yes, the same Giovanna/Joanna who was considered as a second wife for Henry VII of England)? After all, they are the same age, and Giovanna (who had the benefit of not being the daughter of the reigning king, although she might be still less-inclined to trade in her rank as "Her Majesty, the Dowager Queen" for "Her Grace, the Duchess").