Hi Siebe,
you said, Nagasena also denies that the pains of the Buddha were Kamma Vipaka.
From what I have heard the pains experienced by the Buddha were due to kamma vipaka. For example he had a back pain and this was due to him (when a Bodhisatta) breaking someone’s back in a wrestling game. Kamma was also the reason that the Buddha and a group of monks did not receive any Dhana and were offered only horse food for a period of 3 months.
So you may want to double check what has been said by Nagasena.