Is it true that screen vixen Jayne Mansfield was originally offered the role of Ginger? I also read that Tina Louise regrets accepting the role because it ruined her promising film career?

In the pilot, Bunny (Nancy McCarthy) was the Ginger Grant-type character and Ginger (Kit Smythe) was more of the Mary Ann-type character. (see http://gilligansislanders.com/s1ep0.htm).

In the image, Bunny is the blonde on the left and Ginger is the brunnette on the right. John Gabriel played the Professor.
http://gilligansislanders.com/image/s1ep0.jpg

This doesn't answer the original question except to say that Kit Smythe was originally offered the role of Ginger.