So far I noticed that lemang uses sticky rice while ketupat and nasi himpit uses regular rice. Also lemang has coconut milk inside like nasi lemak. Ketupat and nasi himpit doesn't.

But recipe and styles may vary. But from so many I have eaten, the difference is lemang is way more closer to rice desert kuih that you could match with sweet and salty companion dish. It's difficult to pair ketupat or nasi himpit with sweet things like kaya or gula melaka.

I do not think ketupat has coconut milk inside. How to keep the milk retained inside the woven leaves? If they used sticky rice, yes you get the same texture as lemang which uses sticky rice. But no way the coconut milk can be inside ketupat once it is boiled in water.

I can only think that very likely the rice is soaked in coconut milk for some time before inserted into woven leaves. It is very unlikely the ketupat is boiled in coconut milk either as the milk will thicken into gelatinous state before the rice could be cooked.

Unless the weave is well made I do not think coconut is poured in when the rice is inserted into the ketupat shell. But I have seen people adding salt inside while inserting rice in ketupat shell.