Sorry, but It seems you are overreaching Bhante . Your idea’s are consistent with the NIddesa though.
But It seems much closer to AN 9.36 , where the Buddha instructs the following :
a bhikkhu enters and dwells in the base of nothingness. He considers whatever phenomena exist there pertaining to feeling, perception, volitional activities, and consciousness as impermanent, suffering, an illness, a boil, a dart, misery, affliction, alien, disintegrating, empty, and non-self. He turns his mind away from those phenomena and directs it to the deathless element thus: ‘This is peaceful, this is sublime, that is, the stilling of all activities, the relinquishing of all acquisitions, the destruction of craving, dispassion, cessation, nibbāna.’ If he is firm in this, he attains the destruction of the taints. But if he does not attain the destruction of the taints because of that lust for the Dhamma, because of that delight in the Dhamma, then, with the utter destruction of the five lower fetters, he becomes one of spontaneous birth, due to attain final nibbāna there without ever returning from that world.
Destruction of the five lower fetters , refers to attaining the Anāgāmī stage, where they can only be born in the Pure Abodes. Upasiva is talking about crossing the flood , not just reaching the state of Nothingness, which he is already capable of.