some brokers, *if informed*, will require the estate of a non-US person to complete US probate before releasing funds. e.g., Schwab
...
Which ones do according to your knowledge not require that if informed?

No idea, sorry.
I suspect Interactive Brokers UK might fall in that category, but I don't have any information on that.
(their UK division handles non-US accounts for the majority of countries that don't have a local IB division)

So far as possible, our family plan is to avoid telling the broker if the funds withdrawal goes through OK, even if in theory it would be OK telling them.
The reason is that telling the broker can only make things more difficult, never easier. Why ask for trouble, if you're already having the worst week of your life?

Oddly enough, a bigger problem is our account in Monaco, which also holds investments.
The bank we use has extremely onerous terms, saying any joint account is immediately frozen then closed if one holder dies.
If you want to stay at the bank, the surviving spouse has to start a new application (a process of months) and open a brand new account.
Something to do with French law, I'm told. But this bank is also extremely officious. Something to do with having French headquarters, I guess : )
They do not recognize the concept of "joint tenants with right of survivorship" (JTWROS) which is our marital regime.
That is the phrase meaning that the survivor is co-owner of an asset before death of spouse and automatically sole owner after death of spouse without probate or process.

Depending on the jurisdiction, JTWROS might conceivably be another way for folks to avoid US estate tax, but that's just a wild guess of something worth investigating.
Presumably a prerequisite would be that both your broker and your jurisdiction of domicile sufficiently recognize the concept of JTWROS.
Or at least are willing to let it slide. Frequently inter-spouse inheritance tax is zero anyway, so many organizations simply don't care.

Jim