Onam is the time to celebrate after two months of rainy season(miserable time for everyone). Hence you have reading of ramayana during worst of those periods. Main reason, can't work outside due to rains, nothing to eat because water issues, sickness etc. And then Onam comes.... What a better way to celebrate the coming of new period than by wearing new clothes adorned with freshly cut beautiful flowers, houses cleaned as if welcoming a long lost King. So if someone call this tradition a superstition, who cares, I love this superstition!!!
Now about stories of Bali and Vishnu, it may be related to some historic event, but whatever it is.. like all our puranic stories its final aim to focus your mind on God. So your guess on this subject is as good as any one who claims to be subject expert on this. So don't fret over it.
Now about rakshasas and asuras, these are nothing but our ancient equivalent of modern day terrorists and goondas.
About aryan theory, its now well known that its a defunct theory.. (just an old theory). In the olden times, people used to good noble people.. "arya" or "arya puthra(son of an arya)". That's it. May be two thousand years from now, they would say in england, there were some race called "Sir".
