Richard's agreement with Henry VI and Leopold V only lasted as long as the latter two lived. Richard saw himself as beholden to no one but God. As for Eleanor, considering her proximity to the English throne, I'm not sure if such a marriage would be allowed, or if anyone would want a repeat of Empress Matilda.
Considering how much influence Philip Augustus had on Arthur IOTL, and assuming Arthur becomes King of England, if Eleanor of Brittany is going to marry anyone it may be someone of Philip's choice. Someone he could control, like:
Theobald VI, Count of Blois (
https://en.wikipedia.org/wiki/Theobald_VI,_Count_of_Blois)
Peter of Dreux, OTL wife of Alix of Thours, Duchess of Brittany (
https://en.wikipedia.org/wiki/Peter_I,_Duke_of_Brittany)