iCalendar - 3 questions

1 - iCloud vs on My Phone:

Never really having understood what the point is of having 2 sets of Calendars on my iPhone. I use mainly iCloud so does it not make sense for all Calendar events to be on iCloud and not on one's iPhone only?

2 - Can you cut and paste - or merge - the entire contents of a calendar into another calendar?

I seem to have ended up with 3/4 iCloud Calendars - one called Home, one called Work, and one called just Calendar + I have added another Calendar called House ( to do with just events that concern all of us who live in the same house).

Is 'Calendar' a 'Master Calendar'? How would I merge say 'Home' & 'House Calendar' without fear of losing years of data or missing some data from years back on a calendar?

3 - Best way to failsafe back up all Calendars beyond iCloud?

I am absolutely dependent now on iCloud Calendars - has become a very important record, the only record for years - Is there a way of absolutely guaranteeing that the dates and info cannot be lost? Can everything be imported to another Calendar app as a back up?