The EU are only prepared to replace the backstop with a legal guarantee of the same thing; Preservation of the Single Market and no border in Ireland. They have been saying as much since Britain voted for Brexit.
The only real negotiation going on involves changing Britian's opposition to that position. Ultimately if we are to have a deal that does not involve the entire country aligning to the EU, then its to have NI align and have a border down the Irish Sea. Is there any other way?
The barrier to that agreement is the DUP, so Boris either has to have an election to remove his dependency on them, or hope that he gets enough support from the rest of Parliament to push it through. At the moment that is unlikely if the DUP don't concede, so we're most likely looking at an extension followed by an election.
What I think he's going to try though is to agree with the EU at the very last second and then immediately put it before Parliament (hence Saturday's emergency session) - it's not a bad gamble, because it might just scrape through.