I don't think they had the support anymore. Certainly not of the common people anymore and the support of the common people was very important in the Dutch republic. The rulers, including the de Witts lived among the common people. It quite often happened that when the common people were unhappy, they simply attacked the houses of the ruling class and destroyed them. Actualy this kind of happened in this case, but instead of destroying their house, they destroyed the de Witts.
Absolutely not. France and England more or less betrayed Johan. Especialy England, who created a treaty/alliance with him and than broke it and joined France. Actualy the plan of France and England was to depose the regents and install Willem III as ruler/king of the Netherlands as a puppet state (after taking the parts of the Netherlands they wanted for themselves and their German allies). Louis XIV hated the republic (as in a state without a king, although he disliked the Dutch republic in general too) and the king of England was related to Willem III. They would not make a deal with Johan de Witt.