1. Archaeological evidence you seek was provided in this very thread with his post about "Syria".
This is a ancient 2000 year old Vishnu idol found in Russia. This is apart from the 4,000 year old settlement found in Russia with Hindu swasthika.
This is an ancient 2,300 year old Indian statue of Yakshi (mistakenly identified as lakshmi) found in Pompeii f the Roman Empire.
This are murals of Shiva from China,
There is evidence in the form of "GOLD STANDARD" which was the currency standard in India and became a global standard before the US dollar replaced it.
There is evidence in the form of currency called Dinhar to Rupya spread from Arabia to Vietnam, which is based on the ancient Indian currency.
There is evidence spread all over the world, you remain blind to it because you choose to be blinded by your prejudice.
Heard of Peacock in king solomens court ? Peacocks are found in India.
2. The decline of the society and civilization is a long story by itself. Its an entire 2000 years of history.
3. No idea about aliens.
4. Every Hindu knows his history till told otherwise by communists or "secular" Indians that seek to undermine our history and civilizational values.
It is history, but its also our roots that nourish our present.
Maybe not from the sands of arabia, but India is ancient and so is its civilization.