There were approxmately 4500 Japanese defenders on Betio when the US landed. The US had roughly 18,000 men for the landings (a full reinforced Marine division). It is very unlikely that the Japanese could have mustered sufficent forces to overcome the landings even with better orgainzation and communications. There are two reasons for this. The primary one is that the Marines had established at least two footholds on the island by the evening of the first day of fighting. This would mean the Japanese either split their forces or they concentrate on on enclave. Either way, they are numerically and firepower-wise at a severe disadvantage. If you add just US DD fire support an exposed Japanese attack would result in massive casualties even if it succeeds in overrunning the Marine positions in whole or part.

The other problem they face is that Betio is only needed for the airfield. The Marines could (and did) still land on the rest of the atoll from Eita Island to Lone Tree Island and establish bases on the other atolls all of which had minimal defenses. As the US has total sea control they could simply come back and try again while holding the outer atolls and continuing to shell and otherwise harass the now isolated Betio defenders.
This strategy was a big Japanese mistake. Fortifying the entire chain of islands and spreading the defenses more while providing for some sort of naval support like small submarines would have made the island a far tougher nut to crack. By spreading the defenses the other islands could have maintained heavy artillery fire on the one invaded sure in the knowledge that the defenders were well dug in and largely immune to the fire unlike the attackers.
Submarines and more coast defense guns would have made things hard for the Navy off shore. They would be distracted and having to deal with these other islands. The attackers, likewise, would also have to spread their own forces thinner to deal with the defenders. By concentrating their forces on a single small atoll, the Japanese eased the US problem of taking the entire chain.

As for the IJN coming to the rescue, what forces did you have in mind solarfox? The Japanese carrier forces (Kido Butai) had hardly recovered from their earlier battles in the Solomons and their air wings were severely depleted in fighting there and against Kenney's Fifth Air Force in New Guniea. Defending Rabaul cost the Japanese a year in recovering their carrier air forces and even then they were ill-trained and equipped to take on a vastly technically and experiance-wise superior US carrier force.

I don't see the Japanese winning this one.