Hey,
So I'm not the experienced player buuuut I can answer your questions to my best knowledge.
Your first question: Well, it kind of depends, if your Sim has the active trait from the bodybuilder aspirations, they will live looooooong after that message, but usually a few days after the Sim without the active trait dies. I think it takes about 2 or 3 days until the bar goes sparkly and then it's another 2 days. It does depend on how the Sim is feeling as well, though. If your elder is feeling uncomfortable a lot, he might die sooner.
Second question: You hear a sound and the grim reaper appears, I'm not a 100% sure if the camera pans to the dying Sim, but I don't think so.
If the Sim dies while you are on another lot, well you just miss that. I hate that about Sims 4. You don't really get notifications about death.
This leads to your third question. As long as your Sim does not witness the death itself or mourns at the grave they just do not care/ do not know about it. I play legacies a lot and I always have to check if the parents that live on another lot have died or if they are still alive.
Next question: Sims with the active trait tend to live wayyy past the agebar.
Next one: You get an urn if the Sim dies inside and a gravestone if the Sim dies outside. You can do with it what you want in build mode. You can even put it in the Sims inventory.
Last one: When a Sim dies they become a ghost, so if your Sim knows a lot of older Sims get ready for mostly ghost in your relationship panel. They do eventually after A few days/weeks fade. But until then you will get calls and invitations and notifications mostly from ghosts.
I hope I was able to help :)