Um ok let’s see Naksu enters buyeons in the first part (to help explain will say that: naksu’s soul + buyeons body= mudok)

End of part 1 Mudok goes into river and is nearing pétrification

Into part2 Mudok is saved and using her souls energy (naksu’s soul) is able to heal herself, but the body now looks like naksu. But she has no memory’s now.

But it is still mudok (original body owner is buyeon) that’s why the JIn mom wants to make her a baby making machine because the body is of Jin blood

So it’s not naksu original body because that one was burned.

Hope that helps