-Did I forget anything in the above steps?

You don't need to shut down the water mains. There shouldn't be a permanent
connection between your sealed pressurised primary circuit and the mains
supply.

There should be a braided hose connecting the mains to the system. This
should normally be physically disconnected (but frequently is not) and the
valves shut off. When refilling, you attach the hose and open the valve.
When finished, you ensure that all valves are off and the hose is
disconnected.

When refilling, ensure the automatic bleed valve is on, if it has have one.

It is easist to refill with two people. One sits by the filling loop topping
up when the pressure goes down. One goes round opening the radiator bleed
valves. With only one, it takes longer because you need to keep going back
to refill.

-If I drain the system, I also drain/introduce air in the boiler. Is
this not a problem for the boiler? Will all the air be flushed out
when I bleed the radiators?

The majority of the air will come out through the automatic bleed valve and
when bleeding radiators. There is also a substantial amount of air entrained
in fresh mains water. You should, therefore, keep the automatic bleed valve
open for a while and rebleed the radiators a week later after this air has
come out of solution. Also, bleed manually if the system sounds funny, with
clanking and other unusual sounds.

-When I refill the system, do I need to mix some anti-corrosion fluid
of some description with the water? If so, how do I do that
considering my system is fed off the mains.

You MUST install inhibitor, such as Sentinel X100. To introduce corrosion
inhibitor into a sealed system, there are many solutions. You can prefill a
radiator with it before reconnecting it, you can get syringes for squirting
it into a bleed valve. Best of all, when the system is drained down, install
an additive filler point, which is just a short vertical stub in pipe,
preferably near the top of the system, with an isolation valve in the
middle. You just open the valve, insert a funnel and pour the chemical. You
don't need to buy expensive proprietary solutions, but don't forget to close
the valve before refilling!

Christian.