As she will be de facto queen, she might as well have the title formally and take part in the coronation. The Prince of Wales has been planning his coronation for decades and revising his plans for decades. Of course, it will be lavish and, of course, the public purse will pay for it.
State funeral when HM pops her bucket, followed by lavish coronation the next year. I suppose we should all hope that he survives as long as possible before another state funeral and lavish coronation are required.
Probably tantamount to treason, but we could all hope that he kicks his clogs after his Mama's state funeral and before his coronation. Standard practice in such an event is to keep the coronation plans and date and simply slot in the next heir, who, in this case, is likely to live for quite a lot more years than his Papa. That would also resolve any controversy about Queen Camilla.