I’ve never lived in SF proper but, while visiting, twin peaks always seemed like it would feel isolating to live in. Is it possible to walk anywhere from there (in a “daily convenience” kind of way, not an “urban hiking” kind of way).

Not really. Maybe if you live in the bottom half right above the Castro district it's doable (but steep), but definitely not in the top half and definitely not if you're elderly or have any disability. I usually walked down and took an uber back up, back when the VCs were subsidizing the rides.

As awful as it is that's the trade off: if you can't conveniently walk up the hill, criminals and homeless people usually won't bother either.

Ya, this is also a double edge of transit: if you have a nice convenient public transit system, so does everyone else, including criminals. This is why sometimes neighborhoods fight to keep bus routes or light rail stations out.

Even if this might be true in isolation it doesn’t generalise

For example the poor public transit available to developed places such as West Sydney, or Bijlmer in Amsterdam have lead to increases in crime rates

Poor physical mobility for the lower classes leads to decreased social mobility which results in increased crime rates for the larger metropolitan areas as a whole

This is first or second year Urban planning mate

Yes, but Mercer Island doesn’t really care about Seattle’s crime rate as long as it’s own crime rate is low. Local politics is local.