The only thing stopping it would be their putting Tory party interests first.

And that is precisely what the Tory Party exists to do.

But I cannot see how parliamentary procedure and parliamentary arithmetic could work out the way you suggest, Steve, even if there was a revolt by Tory backbenchers. Legislation to change the law in Northern Ireland could only have any effect if it was government legislation. A motion by the opposition, or by backbenchers of any party, even if it passed, would be merely an expression of opinion.

If this was treated as a vote of confidence, and the government fell, Jeremy Corbyn would be invited to form a government. Unless a significant number of Tories supported a vote of confidence for him, he would be unable to do so. A new leader of the Tory party might be given a chance to form a government, but they would need DUP backing to form a government. If they could not, parliament would be dissolved for a fresh General Election.

I can see no circumstances in which it is possible for the present parliament to make the legislative changes you suggest, even if a majority of members might wish to do that. Unless there was to be a breakaway group of Tories willing to support Jeremy Corbyn as Prime Minister, which I do not see as a credible scenario. (It is theoretically possible that an alternative Tory leader could seek allies from other parties to replace the DUP - but I find that highly improbable. Can we envisage the LibDems or SNP cut their throats by doing that? Or even a bunch of Blairites?)