Well, I loaded a save file in Ch 11 in which I'm about to ride the shuttle to Cocoon. I surveyed the map but don't see a cie'th stone portal. Do I have to back track if I want to complete missions before proceeding to Ch 12???
Well if you are keen on completing some early then yes, you must backtrack. Some missions are hard at your current stage. You do not get to go back to pulse till after you complete chapter XII. You get the portals at the start of chapter XIII. I suggest you complete chapter XII first. Once you complete XIII.. the game is over so I suggest beating the game first. You will unlock the last stage of the crystarium which will make you stronger if you get that far.
Those words I was unable to convey... That day... That time... Those thoughts I was unable to carry out... Words and thoughts... The connection between the two... Without words, thoughts cannot be conveyed... Without thoughts, there are no words... They are both as vital as each other...They can never be divided... Like the wings of angels...
In Chapter 13, 2 maps before the final boss battle, there will be three portals. One leads you back to Eden (Chapter 12), one leads you to the final boss, and one leads you back to Gran Pulse, Vallis Media. From there, you can do all the missions. Like the person above me said, it would be wise to beat the game first, as that unlocks the last level of the crystarium. Without the complete crystarium, it would be VERY difficult to complete missions, like 51, 54, 55, 61, 62, 63, and 64.
Completed: Final Fantasy I, III, IV, V, VI, VII, X, X-2, XII, XIII. KH I, II, CoM, Re:CoM.