@Whoopiedooo
I can answer a few of your questions (I'm a lifelong resident of California and, shall we say, familiar with the system).
Would a divorce be under UK or USA jurisdiction, or is that the choice of the person who begins the process?
In order to get a divorce in California, you must be physically residing in the state for 6 months, and in the county of filing for 3. Owning real estate (ie the Montecito place) doesn't meet that qualification. One way around that is to file for a legal separation, there is still residency, but no time restriction. The financial & child custody/support (US terms) rules in a separation is identical to those for divorce. Meghan could leave tomorrow and file for a separation the next day. It could then be converted to divorce proceedings once she's been here 6 months.
HOWEVER, filing here may not be to her advantage.
Posters saying she is waiting for the inheritance, do they mean waiting for KC to die......Or is there something else that would feature in a divorce settlement or child support deal?
In California, inheritances are NOT community property unless they are 'commingled with joint funds' or 'used for family needs', no matter when they are received. If there were to be an inheritance she would be SoL unless Harry put the money in joint funds or used some of the money directly on family needs. There have also been cases where a spouse has left soon after an inheritance is put in joint funds and a judge has ruled that they are not entitled to a part of it due to the timing. As long as Harry (or his advisors) are smart enough to keep any inheritance completely separate from joint funds and he doesn't use any of it, it will not feature in a divorce settlement. But I'd say the chances of that, especially the 'not using it' part, are nil.
An inheritance itself is NOT used to calculate child support. Income generated by it is. Distributions from an inheritance based trust might be (grey area). And judges do have leeway/discretion in cases of extreme wealth. Judges have been known to award a large one time lump sum designated as specifically to be used for the child(ren), as well as ordering ongoing payments.
I'm sure if M is contemplating divorce or legal separation, she is savvy enough to have already quietly sought both US and UK legal advice and will already know all this.