He chose the path that would give Chani children. Also, he bargained with Mohiam AFTER Irulan was giving the contraceptive. There is no evidence that Irulan managed to give Chani an abortifact.......but I would say WoL in a drink right before delivery would be an awesome poison. and no, I don't think an abotifact would be an illuminating poison to the twins. Only an Illuminating poison would do the trick, and WoL was easy to obtain in that location.
Paul did permit this, any other path lead to worse suffering....meaning, Chani didn't have to die right then, but he chose the path that killed her. Why else take Irulan and Mohiam into the desert with the group?
Two babies?
Again he stumbled. Chani, Chani, he thought. There was no other way. Chani,
beloved, believe me that this death was quicker for you . . . and kinder. They'd
have held our children hostage, displayed you in a cage and slave pits, reviled
you with the blame for my death. This way . . . this way we destroy them and
save our children.
Children?
Once more, he stumbled.
I permitted this, he thought. I should feel guilty.
But the Reverend Mother was in no mood for excuses. Irulan received explicit
instructions now to meet this new threat. If Chani conceived, an abortifact must
be introduced into her food or drink. Either that, or she must be killed. An
heir to the throne from that source must be prevented at all costs.
An abortifact would be as dangerous as an open attack on the concubine.
It was not a hajra, a journey of seeking. Paul sought nothing here except,
perhaps, a place for her to give birth. He had summoned odd companions for this
journey, she thought -- Bijaz, the Tleilaxu dwarf; the ghola, Hayt, who might be
Duncan Idaho's revenant; Edric, the Guild Steersman-Ambassador; Gaius Helen
Mohiam, the Bene Gesserit Reverend Mother he so obviously hated; Lichna,
Otheym's strange daughter, who seemed unable to move beyond the watchful eyes of
guards; Stilgar, her uncle of the Naibs, and his favorite wife, Harah . . . and
Irulan . . . Alia . . .
Paul already had all the answers.
"I've made no such decision," Paul said. "What I will do is go into the
desert as soon as it can be arranged. Our child will be born in sietch."
"A wise decision," Stilgar intoned.
Alia refused to look at Stilgar. It was a wrong decision. She could feel
this in every cell. Paul must know it. Why had he fixed himself upon such a
path?
This last quote was after the bargaining with Mohiam, and after the contraceptive. Even Alia could feel the danger....but Paul had already chosen his path. Chani could have lived throught the birth, but Paul chose a path where she wouldn't....for good reasons.