What is the difference between Arupaloka and Nibbana except Nibbana has no incarnation, no dukkha and has Buddhism teaching ?
The arupa ayatanas are the subtle enclaves of the coarse human consciousness. The word consciousness here means the continuum of experience derived from the arising of sensate phenomenon, which is more accurately described using the five aggregate model: with the arising of form, feeling/perception come into play from which two reactions arise: aversion and attraction. In this world/body relationship consciousness becomes a self-replicating repository, and it is in its very ability to self-replicate that one finds a continuum called Sue, Bob or John. Those entities then haul around the regrets of their history - which is akin to hauling around a slab of concrete - and make feeble attempts to offset these regrates by projecting favourable images of a better future. In essence, it is a neurotic consciousness where one creates a stressful here and now. That is the delusion, and that is what Dogen calls The Ten Thousand Things.
This is a very noisy situation - so noisy that one cannot discern or even conceive of another way. Practice aims to reduce this type of noisy consciousness so that illuminating knowledge of another way can be discerned. Once one has reduced a substantial degree of this type of consciousness, you enter into the arupa ayatanas. A comparison between the arupa ayatanas and nirvana only serves a mind intent on food for thought. It gives something for the mind to lick - like an ice cream. It is better to not alight on anything with regard to comparisons.
Q2) Why brahmas unable to hear the teachings of the Buddha at Arupaloka?
They don't have ears... but to bring an answer closer to earth in terms of meditative qualities: it is only the last two arupa ayatanas where one cannot discern a state, a non-state, or neither a state nor a non-state simply because the consciousness there is extremely tenuous. In other words, a teaching cannot be accessed. It is only in retrospect that can one come to understand the last two arupa ayatanasayatanas; when one has emerged from those states, then it is possible, by means of reflection, to know those states. But when you are in those states, you cannot be taught anything. Regardless, the mind is radically changed by such subtle types of consciousness. It becomes a wafer several microns thick, primed to snap at the mercy of the lakshanas.