Retreat in the face of heavy casualties is not a Marine Corps tradition.

Carl Schwamberger wrote:

"So, I'd guess a complete repulse would result in more of the same, a evaluation of what was done wrong and improvements implemented. Keep in mind the Betio attack was not the first or the largest amphibious landing, or assualt, to date. There had already been a half dozen such operations in the Pacific theatre and one defeat would not be a show stopper. In Europe the failure of the Dieppe raid did not bring a halt to amphibious operations. So its unlikely the overall strategy in the Pacific would change. Or even the operational technique. Just the tactics would be modified, much as they were continually improved anyway."

That's it in a nutshell, and it is doubtful whether anyone could say it better.

Solarfox:
IMO, the most likely option would be for the Japanese to leave their pillboxes before the Marines set foot on the beach, and even that is iffy. Since it is your goal to posit a Japanese victory, you might consider that approach. As for the consequences, the advance in the Central Pacific would have been slowed temporarily, but not derailed, as others have pointed out.

JT