The standard sharpening angle for chisels and plane irons is 25°. As you say this is the angle youneed stonework up through the grits to arrive at. Your bevel should be right the way through from the front face to the rear face of the iron or chisel. The lower the grit number the faster the abrasive cuts making low grits ideal for grinding out nicks and chips in the cutting edge
The standard honing angle is 30°. As you say you only need to work the last 2 grits because the bevel itself only needs to be 1 to 2mm initially. It is this secondary bevel which does mist of the actual cutting.
So why have a secondary bevel? Why not just grind everything at 30°?
If you think about it, when you grind at 25° then hone at 30°, giving just a 1 to 2mm secondary bevel, when the blade does start to go a bit blunt it can be "touched up" very quickly by just rehoning the blade. OK, so this means that each time you do it the secondary bevel does grow by 0.3 to 0.5mm or so every time you rehome it, but it does mean that providing you don't need to grind out a chip you should get 5 to 10 honings (shzrpenings) for minimal effort. When you work extremely hard timbers, e.g oak, antique pitch pine, teak, iron, etc. it is often the case that you'll only get 10 to 20 cuts between rehoning. When that secondary be el.grows to 4mm or maybe a bit less, then that's the time to regrind at 25° before adding a secondary bevel. The angles chosen aren't arbitrary. They are meant to allow general use in both softwoods and hardwoods whilst giving reasonable edge life and performance. Well, it works for me and saves a lot of time and effort
Properly honed you should end up with a tiny swarf across the edge which must be removed.
That said there are some chisels which only get a single (less acute) bevel and where a stronger edge is required, e.g. heavy mortising chisels. At this point, though, I wouldn't worry about these as you are unlikely to be using them just yet.
Why flatten the backs of plane irons and chisels?
Mist obviously, in the first case if you don't flatten the back before using a plane iron or chisel and there is any pitting in that back, you simply won't get a consistently sharp edge. Once flattened (to 600 grit) it is only necessary to buff up that face every few times you sharpen/hone with your highest grit paper in order to help keep rust at bay (a wipe over with a rag dampened with 3-in-1 oil ever time you hone or sharpen also helps)
Another thing to consider is that in a bench plane it is the back side which faces the wood, so it needs to be smooth and ideally semi polished. One more thing this does is to enable you to get a tight fit between the back iron and the iron itself (this needs to be a good tight fit to ensure that shavings don't jam betwixt the twain
So flattening the chisel makes the back all shiny? U rub the back and on (as in picture with big old chisel) and work up through grits? Same as getting rid of burr. Do this after honing I guess.
As well as the stone I made from your advice, with wet and dry paper; I bought a diamond sharpening stone kit 400 grit/1000 fine grit. Is that just for honing?