Look at the evidence, only around 93.5% of people who have ever lived have died [0]. With a sample size of 108 billion, I think we can say the data is quite accurate. Therefore I think your chance of living for ever is more like 1 in 20.
That's correct. However, many people live their life as if they have an eternity to waste. Truth is that life is very short, and a technique like memento mori can help us focus on the essential.
I'd say "Carpe Diem" is the message, not Memento Mori.
I'd say that remembering that I have to die one day — and not knowing when it will be — makes me seize the day.