I believe that both the Gilberts and the Marshalls could have been bypassed for the Marianas. The Gilberts and Marshalls were not easily supplied. Fighter raids would have destroyed air power in the Marshalls and Gilberts just as they did at Truk and Rabaul.
to note that the Gilberts had to be taken. US carrier forces were never strong enough to project enough power, nor did they have adequate staying power to achieve uncontested air superiority or... more