I have read some of Prof. P. N. Oak's books and articles. He has done a lot of original research. But unfortunately his research was directed against a particular religion.
Why do we talk about only Mogul invasion?
India has been invaded and ruled by many outsiders in the last 3500 years. One of our greatest kings Kanishka was a Kushan. We were ruled by Kushans for a long time. Please see
The Kushans Kings were Zoarastrian, Buddhist and Hindu. Their coins prove this.
There is a gap of hundreds of years in the Sanskrit literature when the non Hindu Kings were ruling India.
The Sakas also ruled parts of India.
Then the Huns (Huna) invaded India and ruled for a short time in some parts.
The Kushans and the Sakas merged with the Indian population. Even the Huns. There is a unproved theory by some historians that the Pallavas were originally from Iran. This theory was written only based on the similarity of the words Pehlavi and Pallava.
Shajahan was an Indian King. Akbar is called one of the greatest of Indian Kings.
It may come as a surprise to many that the first translation of the Upanishads was done by Dara Sikoh the son of Shahjahan from its original Sanskrit into Persian.
It has been proved beyond any doubt that the Nagaraja temple in Nagercoil was originally a Jain temple. So what? The temple at Bhadrinath changed hands between Buddhists and Hindus. We really do not know whether the present idol is that of Buddha or Vishnu. The temple at Pazhani was founded by Bogar a siddha who was a Chinese.
Indian culture because of its antiquity has absorbed parts of many cultures. It is a culture of acceptance and tolerance. We have become great because of this.
Hinduism is the only religion in the world which accepts all other religions. This is what makes it the greatest religion. If you take away this then you are no longer practicing Hinduism.