At the risk of sounding relativistic, I don't think there are any right/wrong answers to your questions, Yaakov.
You seem to be arguing that Lessing's decorum applies to poets as well as poetry--that the poet must suit the subject biographically whether or not the poem suits it stylistically. Let me respond with an example in which a poet didn't fit the subject, yet produced an elegy that not only captured the moment but also captured the imagination of a world contemplating a different albeit eerily similar moment.
I'm talking about Auden and "September 1, 1939." Seamus Heaney has dismissed Auden as "a writer of perfect light verse," yet there's no better contextualiztion of the Nazi invasion of Poland than as the outcome of "a low dishonest decade," and you'd be hard-pressed to find a more forceful jeremiad against narcissistic self-regard. It's unsurprising that this poem spread like wildfire in cyberspace in the weeks following 9/11. In their guts people immediately understood the relationship between the two September days.
It would have surprised Auden. He left this poem out of his collections--I remember reading it on a handout in graduate school. Simply put, Auden's readers overruled him. (Screenwriters too: this poem is quoted in the 2008 terror thriller Body of Lies.)
You're right, Yaakov. Poems come by themselves. But readers don't "describe the effect." They absorb it, they internalize it, they complete it.