As with any business or organisation, using your business address or email address or other resources provided to you to carry out the functions of your role for personal matters is not appropriate. In this case, the councillor email address implies to anyone outside the organisation that the function is sanctioned or supported by the council and therefore covered by council insurances, managed in accordance with council procedures etc. As you have said, your council resolved not to support this event so use of your councillor email address is inappropriate, misleading and may even be considered fraudulent.
By all means arrange events or carry out actions for your community, but unless sanctioned by your council, you do so as an individual resident.