2. What is the simple mathematical problem? Finding private key if someone reuse k value?
What is that? Is it similar to this:
https://allprivatekeys.com/random-vulnerabilityThen don't worry. Last month I've checked all of bitcoin addresses for this and found like 1000 addresses with this vulnerability and only 3 with balances. All 3 <500 sats. So, you won't find anything new...
Might as well go after addresses with exposed public keys then.