FWIW I'm a lifelong Calif resident who just went through a legal separation & whose DC1 is going through a messy divorce. The financial laws are the same for both.
There is no '10 year payout' per se. It does give one party more leverage to ask for spousal support and a larger settlement but it's not automatic. A lot of it depends on the 'requesters' own financial state and their ability to 'maintain the lifestyle they've been accustomed to'. If M's 'lifestyle' wouldn't take a deep nosedive as a result of the divorce then she probably wouldn't get support or a larger settlement. If her lifestyle has been financed in large by Harry's income/assets and she wouldn't be able to maintain a similar lifestyle on her own, then she probably would be. How much depends on their relative finances. Another thing that plays into it is the date of separation. M would have to 'begin living separately' after May 19 2028. She'd have to be able to establish that she and H were 'living together at the same residence' so it's not likely she'd be spending the majority of her time in the US or Portugal prior to that date.
As far as settlements, we're a community property state for anything accrued during the marriage. Assets owned prior to the marriage generally remain the persons sole and separate property assuming that the spouse's name was never added and no community property funds were added to them. Inheritances, no matter when received, are the recipients sole and separate property under the same rules. Income directly reinvested is also not community property.
So, the inheritances received from Diana, QEtQM, and HMtQ, would probably NOT be considered community property under CA law as long as M's name was never added to them and/or funds accrued after the marriage were not added to them. Chances are those funds were tied up tight in trusts. Whether they were released to H at some point, who knows. But even if they were and he moved them into non-trust accounts, as long as M's name wasn't added they should be excluded. Of course, there can be exceptions but a court would have to decide that on an asset by asset basis.
From what I know of UK law (learnt from MN threads) it's quite possible that M would be better off divorcing under UK law, especially wrt to inheritances. I also have a feeling they probably had an iron-clad prenup (or a postnup). Either kind is valid under California law and would be considered as binding in the event of divorce even if drawn up in the UK, as long as it met CA law requirements. I know that pre/post-nups may be considered in UK divorce, but they aren't legally binding.
I've heard they aren't selling Montecito*. If they don't M (or H) could possibly claim residency in CA for a divorce, should that be 'more favorable'.
*I've snooped on some of the more popular US property sites but so far haven't seen anything that I can pinpoint as theirs. Not knowing the street it's on is a definite hindrance lol.
Thus endeth the lecture on CA law for today. The issues I mentioned actually came into play in my separation and are currently in play my DC1's divorce.