There’s lots to talk about, in the context of this passage.
First, the word that’s translated “preach” is ἐκήρυξεν. This word means “herald” or “proclaim.” The implication is that Jesus made the proclamation of the Gospel – that His death and resurrection opens the gates of heaven – to those who were awaiting that good news.
There’s more here, though. The phrase in the Greek is τοῖς ἐν φυλακῇ πνεύμασιν. Literally, it means “to [the] in prison spirits”. In other words, I don’t think it’s unreasonable to look at this as meaning that Jesus, in the spirit, ‘journeyed’ to “the imprisoned spirits”. This would mean that the force of the phrase isn’t the location so much as it is the state of existence of the spirits to whom Jesus proclaimed the Good News.
There’s lots more good stuff here – words that have awesome meanings, but are translated in ways that we contemporary speakers of English completely miss! – but I’ll stick with your questions and stop here…
Were they uniquely unable to know whether they were saved or damned, notwithstanding that, as you imply, all were saved?
The Church doesn’t have a doctrinal teaching on this question. But, we can infer that, since they weren’t being punished, it wasn’t the “hell” of damnation. Those who lived in ancient times would not have expected ‘heaven’, but rather, merely ‘sheol’ (that is, “the abode of the dead”), so one might conjecture that they may have thought they got what they expected. Who knows?
Note, too, that I’m not implying universal salvation; rather, just that the ones to whom Jesus made His proclamation were those who were heaven-bound!
Why did Jesus think to preach to them if to no others since then? Why would they benefit from it if we don’t?
Again, it’s less “preaching a sermon” than it is “announcing the Good News of salvation.”
What is time to God? What is it to people who are no longer living in time? When is a person dead from a Divine perspective? Unless we know those things, we can’t know when a person’s consignment to heaven or hell occurs.
I’m not so sure about that. The Church teaches the particular judgment, at the time of a person’s death, at which point he knows his eternal destiny. At that point, the person immediately is consigned to hell, or attains to heaven (unless he first needs the cleansing of Purgation).
I don’t purport to know those things. But I don’t know that anyone does.
The Church does, and she teaches it.