This is such an interesting topic. I agree with you on Karma, it alone cannot take one to Nibanna however it helps you on the path.
After reading further from the looks you and I are referring to Two Types of Nibbana. I believe you are referring to Sopadhiśeṣa-Nirvāṇa whereas I am referring to Nir-Upadhiśeṣa-Nirvāṇa. Please see the descriptions below, they are slightly different.
(sopadhiśeṣa-nirvāṇa), "nirvana with remainder", "nirvana with residue." Nirvana is attained during one's life, when the fires are extinguished. There is still the "residue" of the five skandhas, and a "residue of fuel", which however is not "burning". Nirvana-in-this-life is believed to result in a transformed mind with qualities such as happiness, freedom of negative mental states, peacefulness and non-reactiveness.
(nir-upadhiśeṣa-nirvāṇa), "nirvana without remainder," "nirvana without residue". This is the final nirvana, or parinirvana or "blowing out" at the moment of death, when there is no fuel left.
Where I am from, most people are only aware of the later nir-upadhiśeṣa-nirvāṇa "blowing out" at the moment of death, when there is no fuel left.
So thank you, I have learnt something new today which is sopadhiśeṣa-nirvāṇa