Is there any work being done on reverse-transactions in bitcoin?
Yes. They just aren't widely implemented yet.
See:
https://en.bitcoin.it/wiki/Contracts#Example_2:_Escrow_and_dispute_mediationBitcoin has the added benefit that both sender and recipient can choose in advance who has the power to reverse a transaction in case of a dispute.
[...]the transaction could be reversed by consensus?
In principle, any bitcoin transaction CAN be reversed by consensus. Bit since this requires changing the protocol it is unlikely to ever happen for individual transactions.
During the March 2013 fork event transactions were successfully reversed:
https://bitcoin.org/en/alert/2013-03-11-chain-forkMore realistically, you could also set up a multisig transaction that requires the consensus of a "committee" of previously selected mediators.
Why was this block rejected? What in specific made it incompatible with earlier versions of the software?