I don't think there's any need to drain out the X400 before fitting the Magnaclean (except enough to prevent a flood when you cut the pipe) From what I found, the X400 loosens the sediment so it circulates in the water, and the Magnaclean traps it. If you clean the trapped sediment off the magnet, the X400 continues to circulate, loosening more sediment all the time. It seemed to me OK to leave it circulating until the Magnaclean has stopped picking up significant amounts of sediment (though there seems to be a bit more loosened each time you empty and refill, perhaps the air bubbles dislodge it).
If you drain off into a plastic bucket and put the magnet in the bucket, it pulls the black out of the water. To save wasting chemicals, you can then tip the clean water back into the F&E
Another useful thing is to leave the pump on fast speed, and close down most of the rads, opening one or two at a time so the max flow goes through that section to disturb settled sediment (but watch your vent pipe into the F&E to make sure there is no pumping-over). As far as I can see, you can leave the pump running with the boiler thermostat turned down, to avoid over-heating your house and wasting gas on a warm day. I don't deny that a Power Flush would be quicker and more powerful. The great thing about the magnaclean whie you run X400 for a week or two, is that the sediment is taken out of the circulating water, so it can't re-settle while the pump is not running, or collect in quiet corners.
You are supposed to drain and flush the X400 when you've finished the clean, before refilling with X100. I dont know what woud happen if you left X400 in the water long-term. Try to avoid buying budget-brand inhibitor, you never know if it is as good as the leaders....