now that you ask that question, it all puts everything within perspective. here is my theory: Maham knows that Jalal is beginning to stake a lot in Jodha and she learns this upon the shaadi in the serial. as you remember, during the shaadi track, she suggested that Jalal save himself the trouble of the wedding rites (implying that he should just make Jodha his concubine or just 'take' her into his harem) and Jalal glared at her for making that suggestion. then, she watches how Jodha seems to get away with certain offences, even if some of those offences were machinations from Maham herself.
the closest Maham came to actually getting rid of Jodha for good was during the miscarriage track when she framed Jodha and her brothers for Ruqaiya's miscarriage. fortunately for her, Jalal was not using logic and came close to actually executing them. unfortunately for her, Hamida and the qazi had pleaded on their behalf, creating enough time for the artisan who created that box (a loose end on her part) to come and blow everything in her face. the second time was when Jalal had already planned to divorce Jodha and send her back to Amer...well, until Mainavati came in and told the latter she cannot come to Amer as a divorcee. the third (which was purely foolishness on Jodha's part) was when she removed the bullets from Jalal's gun and bro got injured by the tiger, and Jodha chose to tell everyone what she did (even after Jalal told her to not tell anyone what she did) and the harem blamed her, claiming she wanted to kill Jalal.
anyway, for my theory on why Maham chose to use Benazir, Sujamal and Atifa...directly killing Jodha would be social and political suicide. for one, there is the Rajput side who would most likely not take it if a Rajvanshi daughter was killed by a camp they are still learning to trust. in fact, I am kind of surprised that Amer was not aware that Jodha had drank poisoned kheer to save the Shehenshah, as well as the events that led to her going that far. Maham doing that would create a war, one which might be catastrophic for an empire that is still finding its bearings. even if the Mughals had the upper hand, it would have cost a lot of labour because it is likely that it would not just be Amer waging war against them.
secondly, if Maham had directly assassinated Jodha, people would most likely trace it back to Maham and it will end up badly for her. there will be people in the court observing who is allied with whom and who is whose enemy. should Jodha die from any political scheming, people would most likely point the finger at Maham.
again, these are my theories, and I could be wrong here.
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