CHOGM events aren't usually all that interesting. The Queen meets all the Heads of Government individually and then collectively for a photo op. There is then a dinner with HM present and then she leaves while they actually do the talking.
CHOGM is usually over 3 - 4 days with the royals there for one day only - the first one - and gone before the important issues are raised.
Sometimes HM is asked to raise some issues personally with different HOGs to see the way things are lying but she isn't involved in anything important.
This will probably be her last CHOGM so they will make it special for her I am sure (was supposed to be in Sri Lanka but had to be moved and so was moved to London). Future CHOGMs will see Charles and Camilla attend as heir and then as monarch.
As William hopes to be the future Head of the Commonwealth he should be there - regardless of whether Kate is having a baby or not - duty always has to come first and if he hasn't realised that yet then he isn't up to the job, clearly.