I'm Emperor of Alba and besides the Kingdom of Eire I control the Kingdom of Wales(1). One of its Dukes is the Grand-Mayor of Gwynedd, which has built a lot of trade posts and stuff over the years.
The game doesn't allow me to grant him the kingdom title, saying its type is wrong. But as far as I understood, there could have been republics with a rank higher than duke, weren't it?
I could also revoke his title since he was a rebel, but I fear that doing so would destroy his families and trade posts, even if my intention would be to grant it to another mayor, right?
(1) actually Brythoniaid or Bhretain Bheag, can't remember, doesn't matter
Best Answer
As Affine has stated in his answer, you cannot grant kingdom titles to those who are already theocracies or republics. You can grant a kingdom to a lord mayor to make him a kingdom-level republic, and the republic can and will create kingdom titles if they meet the requirements. You just can't grant a kingdom to someone who is already a republic.
I don't know why you would want to grant a kingdom to a merchant republic. It won't help them at all, and if you grant the other duchy to them then they will create the kingdom if they can.
As Affine also said, only 10% of your realm's total holdings can be held by republics and theocracies, so it is best to have many small republics spread out so they don't fight over trade posts and you can rake in that sweet sweet tax.
Oh, and yes revoking their only duchy will destroy the entire republic.