I want to be perfectly sure of one thing so sorry if I insist on one point you stated, which is still not clear to me.
The wiki indicates that there are specific viceroyalty laws in the Realm Laws tab (with Conclave), not just imperial administration law. So you need imperial administration to enable the viceroyalty laws, and then enable the duchy viceroyalty law... only after this, you can grant duchy viceroyalties.
Now, to enable the "Duchy viceroyalty" law, you need the legalism tech at level 8. what's surprising to me is that you have a tech at level 8 by 900 (AND Majesty level 5 since you have Imperial administration). I don't play much with the 769 bookmark so maybe this is possible to get to this tech level this soon, but it still feels strange to me (and I would love to get to trade 8 in 150 years play time as merchant republic).
Final thing, Viceroyalty laws seem to be a Conclave feature. I don't know how it works without Conclave, but maybe there is still a tech requirement (legalism 4 for kingdom, legalism 8 for duchy)