There is a limit to what you can expect from the parish council. A local council (parish or town) does not have a duty to do anything in relation to housing (although it can do certain things if it wishes). It has no control over planning decisions although it has a statutory right to be consulted by the planning authority. Parish plans no longer have any grant support. Creating a neighbourhood plan can be effective, but is a costly and time consuming business and many small towns and parishes will not find it worthwhile.
So I think your main effort needs to be directed to your planning application, which should be granted if it conforms to local and national planning policies. Most planning decisions are made by officers, so you may have limited ability to influence the decision by contacting councillors. You should be able to get some help from local planning officers.
The rules for eligibility for parish councillors do allow someone who works in the parish or owns land in the parish to be a councillor. Nobody can stop any local citizen from standing for election to the parish council. Prior to an election, you can obtain nomination papers from the local election authority, usually the District Council or unitary authority. Officers will help with the process, which is quite simple. Once nominated, you will be included in the poll if there are more candidates than places, or you will be automatically appointed a councillor if there are too few candidates for a poll. It is usually the lack of candidates that prevents a poll.
A council cannot lawfully make decisions except in a formal meeting, for which an agenda has been made public at least three clear days (Sundays don't count) in advance. Only matters clearly identified in the agenda can be decided. The public can attend any council meeting. It is possible for a council to exclude the public during a meeting, but there must be a vote to do this, and a reason must be given, such as the need to deal with information whose publication would breach a duty of confidentiality. This should not happen often. You also have a statutory right to inspect the minutes that must record all decisions made by the council in your parish. If decisions are not being made lawfully you are entitled to complain to the external auditor.