You can chat about all of these things here on the forum. Even those of us who don’t live in the UK, are still willing to help out with beekeeping concepts.
You are correct. Keep all the frames together in the middle of the brood box. He doesn’t sound like my kind of “expert”!
He is just presenting one point of view. I have tried screened bottom boards, and I don’t really like them. My Flow hive obviously has one, so I just leave the slider in the top slot all of the time. I believe that @Dee leaves her screens fully open all of the time. If I did that, my hive would be invaded by ants, and my screen is a little faulty in the mesh spacing, so bees would be using it as an exit/entrance too. I don’t believe that it helps with reducing varroa. I also don’t like using the sticky boards for varroa counts - sugar roll or alcohol wash is much more accurate.
The metal netting (known as the screen) should be the bottom of the inside of the hive. If you are using the white slider, you just put it into one of the slots on the back of the bottom board. Don’t put it on top of the mesh. The upper slot at the back is the position I use, and is also the position to use when harvesting. The lower slot is more useful if you want to trap small hive beetles. However, I have a major ant problem in my area, and if I use the lower slot, ants nest in the gap between the slider and the screen.
You don’t need a dummy board. I have used one occasionally while the bees draw out the outer frames, otherwise they tend to build double comb or lots of crazy comb in the gap at first. However, that gap at the edge is deliberate. It means that you can pull out the first frame without rolling (and killing) as many bees as you would if the dummy board is there, or you tried to cram in an extra frame (some people do).
Hope that helps. Don’t panic, you have friends here. If all you need is information and advice, we can do that. Hands-on help is a bit tougher from 6,000 miles away…