Bitcoin solves the double-spending problem. Trying to solve the double-spending problem outside of the Nakamoto protocol is in essence inventing Bitcoin 2.0...

All these techniques (awemany's penalty tx, zander's double spend alert, avalanche and weakblocks) are all pale and incomplete ersatz of the Nakamoto protocol.

Another option would be to force a tx to cumulate some proof of work "à la hash cash" before being allowed in the mempool. From there, there are many ways to reinforce the security of the tx, but they often necessitate an external oracle, which can be gamed.

In any instance, with the current protocol, no matter the amount of checks the tx has to satisfy, nothing stops a miner from being bribed with a higher fee.

And finally, all these techniques assume that the tx is broadcasted. What about the case wherein miners shadow mine and do not broadcast the double spend? All these techniques immediately become useless.

The person who will be able to find a way to associate a true double-spend proof to the Nakamoto protocol will be a genious.