how many of currently mined bitcoins are lost forever?
In bitcoin nothing is lost "forever", given enough time its possible for one person to own every single bitcoin private key. The time it would take this with our current computing power is incomprehensible, but in maybe 1000 years we will have a computer being able to do this. In our lifetime the technology will definitely not be created but all bitcoins are not "lost forever".
You're talking nonsense. Stop spreading false information. Even if we find a way to easily crack SHA-256 in 1000 years, do you think that by then we would still be using SHA-256? It would be easily increased by tenfold or replaced by something else entirely.
Good luck trying to get a hold of someones private key. If this event occurs Bitcoin would quickly die, as all trust would be lost and the price would plummet.
As for lost Bitcoins, we can't really say. The same thing happens with every currency. How do you know that the address that you're sending to isn't still owned by someone?
There was a thread in which they've tried to estimate the loss by summing up known losses.