Embodied said: “A Deva of the Tāvatimsa devaloka – the lowest heaven of the lower plane thus – reborning as a human, or the opposite meaning a human reborning as a deva in the referred realm, is this possible ?”
In principle, any being can be born in any realm other than those reserved for the Anagamis; see, “31 Realms of Existence“.
For example, a human can be reborn a deva or an animal; a deva can be born a human or animal; an animal can be born a human or deva, even though that is rare.
Y not said: ” The only instance I can see where this may not be clear would be when a deva or brahma ‘descends’ to the human realm to listen to a desana – but then it cannot be said that he is ‘reborn’ here.”
Beings in the higher realms (devas and brahmas) can come to our world any time for a “visit”. In the days of the Buddha many came to hear discourses. It is said that while only one of the five ascetics attained the Sotapanna stage while listening to the first desana, Dhamma Cakka Pavattana Sutta, millions of devas (including brahmas) attained various magga phala.
They can remain invisible to humans or become visible if they want to.
Both devas and brahmas have fine bodies that we cannot see (like gandhabbas). Deva bodies are more dense, but still cannot be seen by humans, unless they “present themselves”.
On an interesting side note, the King of Tāvatimsa, Sakka, became a Sotapanna while listening to a desana by the Buddha. So, he may be still there as a Sotapanna.
Waharaka Thero has mentioned several instances where there were signs that devas came to listen to his desanas. It is still a tradition in Asian countries to invite devas to come and listen before a bhikkhu starts delivering a desana.