Mortality from ARDS is 50% even with ventilators. Source: wife is a pulmonologist and I am a radiologist.

Are there other reasons why C19 patients would need ventilators? Or is ARDS the main reason?

Is that mortality rate consistent across different underlying causes?

ARDS is the main reason. 40% mortality from just ARDS. Mortality goes up with covid19, age, sepsis, and renal failure.

You'll need to back up the claim that "only a few very elderly/unlucky die"

What % of people actually survive after being on a ventilator? Because I've seen various studies and anecdotes of this for C19 specifically, with results ranging from "basically no-one" to "80%, at least temporarily, but hardly anyone is actually out of hospital yet so maybe they're going to die later".

Beyond C19, this study suggests roughly something like 50% long-term survival IIUC: https://www.ncbi.nlm.nih.gov/pubmed/26003390

Ventilators are invasive things that themselves can do lung damage. Fairly common for people to choose not to go on them in end-of-life directives.

Medical system capacity and utilization explain the vast majority of the variation in mortality among different countries, after accounting for stage of the epidemic.

When things are not above capacity, mortality is 0.5% or less. After reaching capacity, mortality jumps 10x or more.

Age distribution can only account for variations within, at most, a factor of two.

So Spain's (and France's to a lesser degree) medical system got overwhelmed that much faster than Germany's or the UK's?

They might have been very late with testing. I know this was barely on the radar in France even 2-3 weeks ago. MRSA has also been suggested as a component.