My gut feeling is you didn't miss anything. I'm not an ER nurse, but with a 2 week history of vague, flu like symptoms you were not looking at an acute cardiac event in the triage room. Or even, a delayed event. You know as well as I do, it's called sudden death, beacuse there was no warning.
I think it is to your advantage that they want you to talk with the hospital lawyers this early. For one thing, the events of the day are still pretty clear in your memory.
It sounds as if they family has made noises about suing because the fellow died and the hospital is being self-protective in getting information early. Remember, anyone can sue for any reason. I think this will protect everyone far more than waiting a couple years and then calling you in to discuss the day.
Read what you wrote, be honest about what you recall and don't make anything up.
Personally, I have great faith in your nursing assessment
abilities because you are so very concerned. Because you know you would not have been casual about a cardiac event. Because you are trying to figure what you missed, what you should have seen to know this was coming, what more you could have done. Really good nurses reflect on their practice, review what they did, evaluate it, and learn from it. You're doing just that. The unfortuate thing is that the really good nurses are so very hard on themselves when they miss something. Good nurses don't take it well when they find they're not perfect, they jump to the conclusion they're no good, when they really are very good, just not perfect.
How do you go on? Well, you get back in triage and you take it a bit slower for a few patients, you realize you're still good at triage, it gets busier and busier, if you get time to think about it you realize you are looking and listening a bit more carefully. Then, on someone you'll catch the subtle signs of something more going on, you'll intervene and be right. You will start to believe you not only can do this but are good at it.
Do some nice things for yourself. Try to be a bit gentler with yourself. Many hugs.