Its not that Jiraiya could take out Itachi, and again.. Itachi and Kisame ran away from the fight since they would melt under the frog belly that Jiraiya summoned if they dont escape. and besides... looking at it.. Itachi pulled of his chakra to use and summon the mysterious black fire. not to mention he used mangekyou sharingan for that purpose and just as he said.. he used a lot of his chakra for that for purpose. so it would be very clear that itachi will loose to Jiraiya at that time... and Itachi is a wise ninja... he dont intend to kill himself just to have the jinchuuriki.. and that is my opinion.

with regards to Orochimaru, he might not be the weakest akatsuki afterall.. though he admitted that Itachi is stronger than he is... but that couldnt take out the fact that Orochimaru killed the third hokage himself. Itachi might kill the third also but remember that Orochimaru even summoned the 1st and the 2nd Hokage.. if not for Sarutobi's sealing jutsu he would have lost to Orochimaru easily.. so i guess Orochimaru is not the weakest...