I think we might be shown in fb that when Jalal saw Zeenat with her finger near Hussain's mouth - maybe when Zeenat was looking at Jalal - Hussain might have licked her finger. After this without knowing Zeenat removed it.
I don't think Jalal has any doubt on Zeenat at all, if he had he would have made sure she is not allowed around Hussain and Jodha.
I think Rahim might find the ungli in the garden and show it to SB, then she might remember Zeenat's finger burn and ask Hakima about it. Show the ungli to Hakima who then might tell that Hussain might have been poisoned with this same poison.
If Zeenat is not sure about which baby she poisoned, if she felt Hassan was harmed due to her poison she would feel worried about being caught. Maybe Zeenat had poisoned Hussain.
Both the poisons might be different cos Hassan had vomiting while Hussain might have got fever.
They might show us that when Zakira and Moti nodded off - Zeenat simply held her palm on Hussain's mouth and nose and killed him this way so there was not vomiting or any track
Could be Hussain did not have any fever actually, it was just Jodha's imagination and Zeenat just choked Hussain in his sleep without Moti or Zakira seeing her