First Question: “Where exactly do “I” hold these nivarana in me?”
All defilements (anusaya, gati, nivarana, etc.) remain in the mental body (gandhabba) and not in the physical body.
– The essence of the mental body consists of hadaya vatthu (seat of the mind)
– When you are reborn in a different realm, a new hadaya vatthu matching that realm is formed by kammic energy. At that time, whatever defilements you have will be transferred to that new hadaya vatthu.
– That is how the rebirth process continues.
– Of course, all those defilements will be changing continuously during a life.
Your second question: “and depending on that answer, how exactly are they removed/sidestepped/dissolved/etc?”
Those defilements are removed by following the Eightfold Noble Path.
– They are removed in four stages: Sotapanna, Sakadagami, Anagami, Arahant.