Why did Colin let Vera go back to the Phoenix Palace first?
Because, he felt guilty.
The reform proposal for the Senate he just brought up would indeed substantially weaken the Governor's influence.
But at the same time, it would also significantly harm the interests of the five Holy Knight Families.
Imagine, if an elder from one of the Holy Knight Families failed to be elected as the Governor after the reform, how awkward that would be.
This would greatly damage the prestige of that Holy Knight Family, and the interests of this family in the Senate would not be guaranteed either.
So, Colin was worried that this proposal would provoke Vera's opposition. After all, Vera was about to become the Northern Duke, and would inevitably consider things from the St. Hilde family's standpoint.