I expect you simply will not be able to grant any more counties to mayors.
The four changes are a fix to a common tactic of making your entire realm archbishoprics and republics to get more income and stability.
Very bad change IMO. Instead of rebalancing the republic and theocray vassals, PI decide to limit their number. I believe they also forbids us to "create neighboring county tier bishops". I'm not really sure what it means, but it sounds really bad to both moddability and gameplay.