note- I won't accept my own answer, as discussion with others here has helped contribute to it.
I'm a lay person.
Having discussed this with people, I now see there is some confusion in the question, which if cleared up, would make for a clearer question. To address the confusion, the question asks.
"When somebody has ARDS, when is non-mechanical ventilation used, vs
mechanical ventilation? (I'm talking no intubation, no sedation)"
One person I spoke to has said
All ventilators are mechanical. And when people say mechanical, hands-free is meant - not manual. Machine operated. And a ventilator would be forcing air in at set intervals. (Note- there are CPAP machines that give a continuous supply of air, but they are not ventilators. https://geekymedics.com/cpap-vs-niv-bipap )
Masks being fed a steady air supply wouldn't be ventilators.
HFNC - not a ventilator. (it doesn't force air in at set intervals).
NRB - not a ventilator
NIV e.g.(NIPPV/NPPV or NPV) - are ventilators
BVM https://www.redcross.org/store/disposable-bvm-bag-valve-mask-adult-size/760002.html wouldn't be considered a ventilator 'cos it's hand operated.
Also, regarding the term "mechanical ventilator" I think it's not really used much as the qualifier doesn't add anything, as when somebody says ventilator, they mean that hands free machine device forcing air in in a timed manner, it's a machine, hands free, not manual. The term "mechanical ventilator" gets 523K results on google. The term "mechanical ventilation" gets 6.6 million results.
The term ventilation is not particular to medicine and means supplying air to some enclosed space. Medicine does have non-mechanical ventilation. A Bag Valve Mask https://en.wikipedia.org/wiki/Bag_valve_mask would be a form of manual ventilation. And as that link mentions, you also have "mouth to mouth ventilation", whicj would also be manual.
In terms of English, machines that don't force air e.g. HFNC would not be ventilation devices 'cos it helps get oxygen in, rather than helps with breathing(getting air in/out).
A BPAP or CPAP machine for sleep apnea machine that does a bit of the work of breathing, could perhaps be a ventiation device. But not a ventilator. A ventilator is capable of handling a person's breathing completely. Like a sleep apnea BPAP machine but with more power.
Specific to medicine, is the term "mechanical ventilation", a form of respiratory therapy, with ventilators. https://www.resmed.co.uk/healthcare-professional/respiratory-care/respiratory-therapy/ And when they speak of non-invasive ventilation, they talk of non-invasive ventilators.
So the question could be split into the question about mechanical ventilators, and a question of, when somebody has ARDS, when are they moved from HFNC or NRB, to NPPV and eventually perhaps to an (invasive) ventilator(i.e. ventilator with intubation).