This “seating” equals being sitting in the throne as King of the Kingdom of God. (Psalm 110:1; 1 Corinthians 15:24) How could Paul know for sure that, at the time of writing of his letters, Jesus Christ was already sitting in his throne and ruling as King? Naturally, because Jesus himself had disclosed that already in the Revelation given to the apostle John.
Just in passing, let me just say this. After Christ ascends to heaven with his sacrifice he remains inactive for the most part until 120 years before the 2nd coming. The 2nd coming is characterized by a preaching work and ingathering work that is done ahead of time in preparation of the 2nd coming, much like John the Baptist was sent out to preach before Jesus started his ministry. Daniel 12:1 thus speaks of "Michael standing up" at one point. Michael standing up represents Christ becoming active in the world again in preparation for the his 2nd coming when he returns to the earth as a man.
Having noted that, your presumption that Christ sitting at the right hand of God is connected with his kingship is not consistent with when he takes up kingly power per the gospels, which is at the time of his 2nd coming and not until Satan is cast out of heaven. I reference Luke 19:12 where Christ has not yet secured kingly power at the time he sends out his slaves:
"12 Therefore he said: “A certain man of noble birth traveled to a distant land to secure kingly power for himself and to return. 13 Calling ten slaves of his he gave them ten mi´nas and told them, ‘Do business till I come."
Chronologically, the sending out of the 10 slaves to do business occurs during his "parousia" and is linked with 1914--that's when the 10 slaves were sent out. The basis for that reference is the parable of the vineyard workers where they work in the vineyard for 11 hours. A half hour is 3.5 years so an hour is equivalent to 7 years. So the 11 hours of work amount to 77 years (11x7=77) from 1914 to 1991. The "last hour" or the "12th hour" during which the workers are paid from the last to the first would be from 1991-1998.
But the main point here is, that at the time the slaves are sent out, Christ had not yet received kingly power. When he returns though to pay the workers, he is king. This simply means that he obtains kingly power at the time of the 2nd coming after Satan is removed from heaven. He remains at God's right hand until his enemies become his footstool. That means until Satan is cast out of heaven and down to the earth. Thus I see some confusion potential if we try to have Christ beginning his absolute reign in heaven prior to the 2nd coming.
Just some thoughts to consider.