Synonymous
They are the same. There are already people in the Kingdom of God/Kingdom of Heaven.
I agree. Jesus talked as if the Kingdom was already his before his ascension (Luke 22:15-16 fulfilled in 24:41-43;) and he said he would sit in his throne after he came with his holy angels, which occurred around A.D. 70:
"When the Son of man shall come in his glory, and all the holy angels with him, then shall he sit upon the throne of his glory:" (Mat 25:31 KJV)
The book of Hebrews implied Jesus was already sitting on the throne:
"But unto the Son he saith, Thy throne, O God, is for ever and ever: a sceptre of righteousness is the sceptre of thy kingdom' (Heb 1:8 KJV)
Philip