Even though the current regency discussion is not directly related to the topic, the way I see it, their is one goal:
Make regencies desirable to play as a player.
From personal experience, I don't care about being a regent for an AI liege. There's not much to gain from it and the mechanics are not engaging.
In order to change that, make playing as a regent the same as playing the current ruler. You can alter laws, you can declare war, take titles (except your ruler's titles. This would need a plot, which has increased success chance in absolute monarchies), etc.
This also means, that as long as the kingdom has a council, you will have to deal with it. You cannot change council power and the underage ruler can influence the other councillors to vote in his favour (if they are interested at all)
What follows is interesting: You now have a balance, between absolute monarchies and council power. In absolute monarchies, you have absolute power, as long as you are a capable ruler. But having a regency can be dangerous if you chose the wrong regent, as he now can act freely, without the council to control him. On the other hand, if you have increased council powers, you have to appease the council during your reign, but they also keep the regent in check during a regency, which makese it more secure.
That would be an amazing regency system.
I like the idea you gave here. The main problem I thus see is the AI. Paradox ai isn't neccesarryly bad, but it's also nothing to write home about, especially in CK2. We should keep in mind, that we haven't seen any CK3 AI yet, but I really hope that it will be more "competent" in lack of a better word than the CK2 AI.
I mean this in a technical sense, not as beeing the characters being more succesfull in their plots or whatever, but what is mainly lacking in the CK2 AI is a goal or drive mechanic. Up to this point, the chracter AI does everything based on some factors, mainly how much they like other characters and how powerfull they are. Just a quick example:
A vassal has chance X every month to start a plot. If this happens, the available plot with the highest factor is selected. (kill target Y if character dislikes them or gains title or raises in succession, modified by some value for greedy or ambitious people and so on).
This system is totally chance driven. After a certain time, the variables are considered and a random chance is generated.
What we need for interesting characters, good regents and good plotting is an AI system which isn't totaly chance based but formulates a (temporate) goal for the characters.
Example:
Vassal Vassili has come of age and has inherited the lands from his father(1 province). What does Vassili strife for? Is he ambitious? he might want more land or a higher rank (or both). Let's assume vassili is unmarried, ambitious, greedy and has some decent stats in diplo and intrigue, normal in martial, the rest is not great. His main goal is to aquire a new province next to his own land, that would be enough to become a duke. He has 2 provinces to choose from (as the duchy has de jure 3 provinces, and he needs at lest 50% to even think about becomming duke). So his main drive is getting to become duke. First temporal drive is to get one of the two provinces. Because he is greedy, the richer province is more interesting for vasilly. because he is best in diplo and intrigue he will consider mainly diplomatic actions or plots to solve his drive. Such a diplo action could be, to marry the daughter of the current holder of the other province(avieving this is another temporal drive, impressing the daughters father to allow the marriage), to get the weak claim on his side. then killing of her brothers because of the intrigue (another drive here). Or he might search a favor with their liege to get the title, or .... .
As you can see, and goal driven AI makes for more complex and interesting characters. If we don't have these, then a regency isn't that bad (or good) in any case, because the AI regent won't really do anything, except if he hates the current heir.
Another short example:
The daughter from earlier: Daisy. she is also ambitious and has good intrigue, the rest of her stats are nothing great. But she dislikes her family/dynastiy and hates her two brothers. As she is a women, she knows she can't inherit directly, but gets a claim if all her brothers die. As she can't hold power directly, she needs someone who can get her the power she want's and the best thing she can do is marry. so if she learns from vassili's ambitions, she tries to get that marriage happen (positive factor for marriage request acceptance, based on how much her father or direct liege likes her), and then helps vassiliy to get the province. depending on culture, laws and so on she might even request then to rule the province as vassilies vassal, or one of the holdings in it. Their heirs will get bot of their lands nontheless.
I think I got the idea across, and won't go any deeper into the matter here.