Moriarty is maybe the most honest character in Sherlock’s mind palace. He calls the bride case as bullshit. He knows whether or not he survived his suicide isn’t what Sherlock is concerned with. He knows Sherlock is really only interested in one thing (John). He knows being heartless is what’s keeping Sherlock down, is his weakness; and he knows only John can save him. It’s funny, but fitting, that Sherlock would choose his archenemy to deliver the most brutally honest revelations about himself.
MP!Moriarty doesn’t lie. So when he says "It’s [John] I worry about. That wife…John Watson is definitely in danger,” we should listen. HLV shows us that Mary is a threat to Sherlock, but she swears her actions are to keep John, it’s all for John, she’ll do anything for John. Liar. MP!Moriarty knows that Mary is a direct threat to John. Which means Sherlock knows it too.
John would probably die protecting Mary, Baby Watson or Sherlock. Possibly all three of them. Seems a fitting death for the man.
If he were to, yeah, it would definitely be for them. As a consequence of his love of danger and adventure could also be interesting, but like with Clara, could veer into hubris or otherwise blaming the victim (though Clara’s arc fortunately rejcted that thoroughly).