JimWhoKnew said:
We should be able, at least in principle, to construct a coordinate chart in which the metric is non-singular at the event horizon
Sure, at least three such charts have been known for decades and are well covered in the literature: Painleve, Eddington-Finkelstein, and Kruskal.
JimWhoKnew said:
and the simultaneity (constant ##t## spacelike hypersurface) that includes our given event, precedes the formation of the horizon.
"Our given event" where? If you mean an event on the worldline of a distant observer who stays at the same altitude above the horizon, no, your claim is not correct; in any of the charts I named above, you will reach a point on the distant observer's worldline where the constant time hypersurfaces now cross the horizon.
JimWhoKnew said:
ll events on an EH are either future-like or spacelike relative to the world-tube of an eternally-external observer.
If by "future-like" you mean "within the future light cone", yes, this is true. However, it doesn't mean what you appear to think it means.
JimWhoKnew said:
Unlike coordinates and simultaneities, this is an invariant property of the spacetime in discussion. In that sense, the claim that EH hasn't formed yet from the perspective of an external observer (at any proper time on her wristwatch), is just as good (or bad) as the claim that it has.
In the sense that both claims are meaningless, yes. The invariant property of the spacetime that you reference is valid, but again, it doesn't mean what you appear to think it means. And it certainly does
not mean that this invariant property is a valid reason to claim that the black hole can't form at all.
Indeed, you used the phrase "formation of the horizon" (I quoted it above)--but that only makes sense in a model where the horizon, i.e., the black hole,
does form. And such a model is self-consistent--nothing you have said implies otherwise, and, as I pointed out in response to the OP just now, such a model was first published in 1939, so it's not a new idea--and it has the invariant property you describe. In other words, a black hole can form and produce a spacetime with that invariant property.