I need to replace an immersion heater that's been in the cylinder for
over 40 years. It looks well crusted in place. So, how much force can
you put on it to unscrew it without rupturing the cylinder? Is there
anything I can do to ease it, or is brute force the way to do it?

Also, the cylinder is full of hot water. I'll need to drain the cylinder
through a hose at the bottom. Will an ordinary garden hose be okay with
piping hot water, or do I need to cool the cylinder down first by
running off the hot water?

You can tell that this is not a job I've done before.