I hope HM steps down to allow Charles to ascend the throne, rather than him ascending upon her death. It would be such a sad tiding over the coronation ceremonies! What are the chances this might happen? There's already talk of changing the anthem, but I don't get the sense that HM wants to move aside.
She won't abdicate. She believes that only God can take her from her position.
The Coronation normally takes place at least a year after the death of the previous monarch and thus the new monarch should be past the worst of the sadness over the death of their parent and the mourning period will also have ended.
I have read that the government (this was in the 90s under Blair) wanted to have a shorter period of time between death and coronation than Elizabeth had but the main reason for the length of time was that George VI died in February so the coronation couldn't be until February the following year and they wanted it in the summer.
As for the length of the ceremony - what can they cut out - the official swearing of the oaths of fealty - so no William, Harry, Andrew or Edward swearing allegiance to Charles (assuming Harry has a title by then). That has been a major reason for the length of the coronation in the past as it was the public acknowledgement by the nobles as to who was the rightful king but there is probably no need for that anymore. Would the PM and the Archbishops still publicly make their oaths again when they have done so in private when taking their offices?
I can see an ecumenical service taking place in Westminster Hall but as Charles or William would be not only being crowned King but also as Supreme Governor of the Church of England then the religious ceremony is very important. They could cut out the communion - and have them do that privately but it is a very public sign that they met one of the crucial determinants as the right of the monarch to be the monarch - that they are in communion with the Church of England.
Charles did say once that he would like to be 'Defender of Faith' accepting the idea that all faiths should be protected not just the CoE but he has not reiterated that since it went down so badly with the CoE and the general public. I think he will stick to Defender of the Faith now.