All decisions of a parish council must be made in a properly constituted meeting, i.e. agenda published 3 clear days before the meeting, meeting held in public etc. etc. So no, a decision made via email is not valid unless your clerk has delegated authority to make a decision (usually granted with a requirement for consultation with one or more councillors, but not always), then the suggested approach is likely to be open to challenge. If you're planning a coronation event, you do still have time to convene an extraordinary meeting to agree the proposals before the date of the coronation weekend or at the very least agree to delegate authority to make a decision.