Christie’s goal is the re-election of Joe Biden. He is trying to damage Republicans and the Republican brand to peal off “soft” GOP voters into not voting GOP. You can tell this in the things he has said and done since his “campaign” began.
No, his goal is to keep Trump from being elected.
Any GOP candidate, except Trump, will beat Biden. The second it looks like Trump won’t get the nomination, Biden gets the boot. Biden can beat Trump, but no one else.