There people that gravity doesn’t exist and it’s some kind of buoyancy. Death as the final end of existence? There are many religions that claim that isn’t true. Nowadays it’s even harder who wrote what but next week if I don’t find that text again, can I be sure it was written or could be just in a dream?
1. yes they are objectively wrong, a persons belief does not need to be tied to a fact
2. death as in death of the body, it's very much inescapable
3. the last part is just uncertainty, hardly an argument against objective reality
Ad 2, How do you define it, precisely? Today we have the technology to keep the body alive even if it would stop functioning long time ago on it's own. Is the person still alive? Is the body still alive? Some bodies need permanent technical devices to be able to live, yet they can reach high age. Is it cheating or not?
Let me be annoying and tell you about your second point as I don't really understand your first one. Your perspective is that of a living looking at a dead body. Death itself though isn't an experience of life, Meaning that we can't really talk about what happens after we die. We just don't know and we probably can't know what or how death is. So any answer goes, really. I mean you are right uncertainty doesn't exclude objective reality, but my question in an argument would be: What do you even mean by objective reality?