New 125 Westport hits dock.

Sounds like the cables did not fail but the Mr. Kotter pin or C clip was worn or missing and the cable came off the pin/stud. All ways eye up the cable connections at the motors while checking the oil...the helm controls once a year, of course , IMO.

Ok . I'll say it, especially on old junk like my boat! Lol...

My worst nightmare, lose control and speeding up.
On airplanes we practiced the worst case scenario in flight simulators:
On takeoff, when you are heavy and most vulnerable, an engine would go into reverse: No time to analyze, talk abut with the other pilots, or call for a check list: You have 2-3 seconds to cut the fuel and shut the engine down, if not, you will die and so will 500 pax,
If the above Westport crew had practiced the same and hit the kill switch immediately, damage could have been minimized. (Could have, should have etc., Easy to be Monday morning quarterbacking)
On my boats I have been changing gear and throttle cables before anything was wrong or worn, strictly preventive maintenance. Garage door openers not a factor, yet.

We did too, back in the DC-9 days with external buckets. (Not quite 500 people though, more like 130) On the modern jets no more. Un-commanded reverser deployments do not appear to happen the way they are designed now, and we no longer train for it.

Now, an FMC failure is a whole different cup of tea. THAT is scary.

Aye, the most deadly Un-commanded reverse was Lauda Air, B-767-200 during Cruise. Everybody died.
The jets I trained for in the simulator was the B-747-100/200.
Fortunately it never happened, but with 3 pulling forward and 1 in reverse you have a few seconds to react and shut it down.
My fingers are still close to the red stop button on the Yanmar 6LP-STE when we are close to the dock, otherwise I be like on You-Tube, or worse, on the CNN Evening News.

Look closely and you can see two spring lines ripping cleats out of the dock

Yes, on the 74 especially 1 or 4 doing that would be a serious problem. Pratt put interlocks on the 76 engines after the accident. Not sure how my GE's handle this but again, we don't train for it.

On yachts. are any components, such as control cables, replaced on hourly or calendar schedules? Or only on condition?

You're not going to find physical cables due to the length of the runs on a vessel this size. You usually see those up to maybe 70' MY's at the longest. They're generally changed when they start getting stiff.

With electronic controls, you usually don't change anything on a schedule. However, you may upgrade an obsolete version after 10/15/20 years when it starts giving you trouble. However, it seems like the industry has settled on Sturdy (OEM CAT and MTU in a lot of applications), Rexroth (OEM MAN and some MTU) and ZF as the top 3 contenders, and glenndenning as the factory CAT 360 controls. However, hardly anything has changed in those 3 brands in the past 10-15 years. I've had my share of electronic control failures, at least 10 different yachts. But I run 100-150 different yachts a year, and a lot of new ones and on those a lot of times it's an installation issue (human error).

Hynautic (hydraulic) on a lot of older/larger yachts. Which is a good/reliable system. However, it has antifreeze running through the lines and I've run a lot of 10+ year yachts that the system should have been serviced and hasn't. Very stiff controls for example, where the antifreeze needs to be flushed, filter cleaned and redone...……..

Is there an emergency kill switch on the throttles or near the engine controls? One that would be common to all the engine control quadrants?

Per jets, on the G550 we just treat a reverser deployment as an engine failure, with an auto idle and an emer stow. Simple.

So does anybody have a factual update on what went wrong?

I've run A LOT of yachts that didn't have engine shut downs on the remote stations.

Yikes.

The normal system on the 125 does have stop buttons on all four stations.

I've run a lot of boats that don't even have controls for a station, only a joystick.

So no excuse for losing control of the engines..?
(Unless the stop buttons also went tits-up, as in major electrical failure)

So we have many great ideas based on experience as to what may have happened - but what did happen? Did the captain have a giant heart attack and fall on the throttles - that could have happened too, no?