Do I understand correctly that ン signifies the sound echoing / fading to silence?
While I often feel like the sound is fading out or terminating when I see ン, this has nothing to do with that function. This is what we hear. It's like the english equivalent of "ng" sound.
"Echoing" here is to extend the last sound, right? If so, it's the function of ー.
It's true that ン is likely to be placed at the end under the situation you stated. Since many animal onomatopoeias contain ン at the end in the first place, it might be related to the way Japanese perceive sounds.
By the way, しーん as the SFX of manga was first introduced by Osamu Tezuka (Note: This isn't the first time the Japanese used this word). キーンコーンカーンコーン, the sound of Westminster Chime, was first used in 1954 as a chime in school. So this word shouldn't have existed before 1954 because few Japanese must have known its sound.
This question might not make sense, but in these SFX are we supposed to actually "hear" that ン or does it exist just to show the sound disappearing? For example, when seeing キーーーン on the first image, should I have kiiin "playing" in my head or just kiii that gradually fades to silence?
We hear the sound of ン.
Having said that, these things are just SFX, so that I rarely read and listen to them seriously while reading manga. What I do instead is just recognize the word and the situation.