In proportional representation, who do I call when I have a local issue that is in a federal responsibility?

You still have a local representative under STV...

If you propose a mixed system, most are local MP with a few extra spread out amongst the other parties, what ridings do they represent?

By "mixed system", I'm interpreting that as meaning MMP. The party MPs represent all of Canada. This is unlike FPTP, where the representatives actually might not represent the communities they were elected from (because they can win with less than half the vote).

There is no perfect system

But surely, we can ensure that every vote counts? This is a fundamental tenet of democracy.

proportional representation will always over represent the needs of dense parts of the country.

How? Proportional representation literally means that virtually every vote counts to the results of the election, and nobody gets more or less representation -- exactly how a healthy democracy should be.

Why does each candidate need to represent a riding? Have the ridings each have their own representative for local issues, and then have the extra MPs represent the country as a whole.