No substantial decision can be lawfully made by a local council unless it is meeting formally and has issued a summons to councillors and made the agenda public for the stipulated period of three clear days. Councillors have no powers outside of properly convened meetings.
Planning issues often do have tight response deadlines because of the performance targets set for planning authorities. But that doesn't alter the position. A local council wishing to express a view must hold a meeting (with appropriate notice) within the deadline in order to decide on its comments. If necessary, an extraordinary meeting can be called, or the council can appoint a planning committee and instruct the clerk to arrange meetings when there are matters to decide. Again, proper notice must be given.
You could contact your district council and express dissatisfaction, indicating that it appears to you that comments submitted by the parish council are not based on any formal meeting of the council, and therefore are not the views of the council, but merely the personal opinions of the councillors.