30% gradients aren't that common in the UK
Regardless of riding position (which can change on the flat, as well as on hills - mine frequently does), riding up hills is the same as riding on the flat, except that you go slower for more effort. As long as your aerobic system can cope with sustained effort, that's all that matters
Indeed - they are all paid to go on training camps in Majorca or Tenerife and not all of us are that lucky, which means the only option for a lot of people is to improvise. Which is why you can't really compare how the pros train to how the rest of us train.