3. The estate of a UK domicile individual holding US domicile assets directly, worth over $60000, will have to file a US estate tax return, but can claim rights under the treaty reducing tax to zero.

Though I don't argue with anything in your summary for UK individuals, it's worth noting that the treaty doesn't reduce the tax to zero, it mainly just changes which government gets the tax money : )
Such a person does appear exempt from US estate tax up to the high US-resident threshold, but the same assets still trigger the UK inheritance tax which is almost the same.
Basic rule 40% tax on everything over GBP 325k.

I am idly curious about what happens with UK non-dom owning US assets. e.g., does that treaty provision apply to them?
I suspect not, as the phrasing in that summary says "UK domiciliary".
By way of background, a UK non-dom is someone who is resident in the UK but not (in the specific UK legal meaning of the term) domiciled in the UK.
Few countries make that distinction, but it's a big one.
In effect, a non-dom may be living in the UK all year every year, but generally pays tax only in income brought into the UK, not on worldwide income, roughly till they have lived in the UK for 15+ individual years.
After 7-12 years a five-figure flat annual tax is also paid, but for the typical rich non-dom that's not a big deal.
A UK non-dom resident generally pays inheritance tax only on UK assets, which I guess (?) would not include shares of US stocks (especially if held at a non-UK broker).
But if the treaty doesn't apply because they are not technically UK domiciled, they'd be in the same boat as other non-US persons around the world, meaning the US would want full US estate tax on US assets.
i.e, generally 40% tax on all US assets above $60k.

Jim