(1) it is mostly the products of corrosion, where your steel radiators are "rusting" away (it is not exactly brown rust, unless you have a lot of air in the system, but that will do). It is mostly black iron oxide. Because you have at least two metals in your system (usually copper pipes and iron in the steel radiators) you will get corrosion even if it does not contain air.
(2) A system which is old, or vented (has a feed and expansion tank in the loft), or neglected (has not been kept well-inhibited) will usually have sediment in it.The more of these conditions apply, the worse it will be. Eventually it causes blockages.
You will find you need to vent the radiators because the corrosion releases gas. You may find the water is visibly black. If you take a radiator off for decorating you may find black sludge comes out of it.
(3) yes. Bale out all the mud from the F&E and sponge it clean before you start, and if the system is obviously very dirty, drain it out and give it a fresh-water flush and rinse just to get the loosest stuff out. Draining is more effective if you fit a full-bore 22mm hose valve on a convenient pipe, the water gushes out faster so carries more muck in it. If you put the valve on the Return side of the boiler it will not get so hot and will last longer.
If you have actually got a blockage that no water circulates through, X400 will not remove it as it cannot flow through the blockage,so the pipe will probably have to be cut out to remove the blocked part.
A Powerflush is more effective, but costs hundreds of pounds,while you can do a chemical clean fir £15 plus a few hours of your time.
(4) make sure that air is not getting into the water, e.g. by pumping over or suction side of pump leaking,or a water leak which is constantly being topped up by fresh water
Then, after you have given it a good clean, refill with a good Chemical Inhibitor such as Sentinel X100.If you can do basic plumbing and afford £100, fit a Magnaclean too. It will capture circulating black particles before they are able to accumulate into sludge or hard sediment,and I promise you will be amazed and delighted at how much it traps.
Fernox is another top brand
I am just a householder