That's the most accepted theory. The kata originated from Matsumura (based on a Chinese form?) He taught it to Kyan and Itosu. Itosu taught it to Chibana and Motobu. Motobu (according to the only source I've found) taught only naihanchi shodan, though he likely knew all three. Motobu's version starts stepping to the left. Shimabuku, who studied this form with both Kyan and Motobu, adopted Motobu's method of starting to the left and of only teaching shodan.
Are you really trying to contradict Isshinryuronin regarding isshinryu??? I started in 1966.
Both your points are wrong. As mentioned above, Motobu moved to the left first, though his "style" is not too common. All versions of naihanchi shodan that I know of use the same main movements and are of the same length.
Isshinryu's stepping to the left is not "too far removed" from other styles, as you said. It is of minor importance as the form is performed in mirror image. It doesn't really matter if you do the first half going to the left and the second half going to the right, or visa versa. And nothing prevents this style's version from doing the nidan and sandan versions as you claim, if Shimabuku so desired. Your assertions are completely unfounded, to put it kindly.