The answer to all your questions is "yes". This
link provides the clearest instructions I know of.
Using this method, the area that can be covered in one go is quite limited. I've created a "local" map which I use most of the time and is enough for a typical 100+ mile ride from home. If I go a long way in any one direction, or ride in a different part of the country, I need to plan ahead or I'll find myself looking at my route against a blank background. I have about 20 downloaded maps in total, but I've probably used fewer than half of them.
When you submit a request, you're put in a queue. So far this year the turnaround has been very quick, but last year there were times when the queue was very long and it took several days to get my map.
There is a possible alternative. Some technical whizz on "another forum" posted a link to a stripped-down version covering the whole of Great Britain which is still small enough to fit on a 520. This was created for the benefit of people doing LEL, and I grabbed it while I had the chance. It does work, and clearly there could be times when it's extremely useful, but on balance I prefer to see the detail which the well-known downloadable maps have. I'm not sure if that link still exists.