It is a very interesting question set, and it could be solved with the most important technology ever: the very blockchain.
This additional solution could be added as part of the proposal: the user can create a report, get the hash value of the report, and send a transaction with the hash of thier report in the data field. Even a smart contract is not required!.. look at this testnet transaction I did to myself with the data 0xc3e24ffb055674ff4c3a053a57386262d25846d988151e887acb48652483d440 :
The order in which the reports are confirmed in the blockchain may solve the potential dispute over who was the first on what, then each user without rush can file the original report to a jury channel in discord, toghether whit the transaction with the proof of author rights of the report; and then the jury will evaluate each report deciding for a winner.
The signature would no longer be needed coz, the user already had to sign the transaction to file his report.
An idea to develope a little bit more and then to include in the proper SIP. I hope other members on the forum can check these mad ideas.
As regarding your question N°4, the winner reports should be the most useful ones, so the winner reports are expected to gather the largest amount of bugs in them.
Sadly if there are allowed more than 3 prizes, the proces turns too complicated.