I know this post might cause some controversy, but whether it is going to be saved or not, it's always worth trying!
I don't think there is anything controversial about asking spotters on an aviation forum for help to save an airport!
However, I do think these claims need to be put into perspective:
1) Population alone is not sufficient reason for an airport to be viable. We in Coventry also have a population of c. 300k and many of use would love to see our airport back having commercial flights, but we understand market reality. Also, if you look at airports across a wider field, I don't think there is anything to suggest that 300k is anywhere near a sufficient catchment area for a thriving airport with regular regional and international service.
2) The airport was far too reliant on one operator, Air Southwest, who often ran flights from PLH combined with NQY, putting in a stop on a route which really should have been direct in order to be competitive.
3) The runway was not long enough for larger aircraft, nor was there anyone willing to invest in extending it. Hence far more traffic went to Exeter, which is otherwise a smaller city.
4) PLH was also subject to the same factors which afflicted other UK airports, namely high fuel costs and the double dosage of APD. This was even more stark at an airport where virtually all routes were domestic.
5) PLH was also too dependent on links with London, an air route which could not compete against untaxed and subsidised rail services.
So in many ways the demise of PLH was a case of natural selection, with or without the local scandal that you allege.
Good luck with your campaign, but remember there is much more to this.