simple answers on bee-friendly gardening?

Hmm, Simon, I think you are worrying too much about the complexity of Nature and how effective your own garden would affect local wildlife etc.
If you grow the much cited plants that are known to be beneficial to Insects, animals and birdlife then you will see the positive proof by the number of visitors attracted to your garden which are actually using the plants as food and shelter. One garden full of such beneficial plants will not make much difference but. collectively, along with the millions of gardens in the UK it will make all the difference.

While Bees are the prime pollinators in our eyes there are other unsung hero's that do just as good a job such as the 7 thousand varieties of Flies, the 9000 species of Wasps, Midges, etc.

You're right, you're over emphasising and simplifying the situation. The Farmer isn't interested in the actions of you, me, or the insects in our gardens. He is concerned about things on a grander scale such as the weather, water supply, etc.........while we are worried about the chemicals and pesticides he uses to produce a maximum profitable crop or crops in the year.

Firstly, as gardeners, our responsibility is to stop overly using pesticides and other chemicals and, if driven to, use them sparingly and at the correct times when insects are not busy. Grow the right plants, be careful in the use of chemicals, etc and you will have an effect on the local wildlife. You might not see the Butterfly, Bee, Wasp, Fly etc that's on it's last legs in Natures struggle for survival which then finds the pollen of the beneficial plants you have grown and were able to feed and find the energy to survive...........that's the important unseen aspect you never know about.
When I need some perspective on Life, I look up into the skies with my Telescope and I know that what we humans do is really not going to have any effect on the "bigger picture"..........the vastness of the Universe where we are smaller than a grain of sand. So worry, not and enjoy your garden and the wildlife that uses your garden as a larder and a sanctuary.