I think that Regal Degree is too strong. I suggest nerfing it to to a Throne Room:
"Choose a player. He reveals cards from the top of his deck until revealing an Action card. They put it into their hand and discard the rest. Play the revealed Action card twice (this is that card until it leaves play)."
I think that choosing a player gives you more flexibility. I am not sure about the rules but I think that the "this is that card until it leaves play" wording would imply that if you hit a Duration card Regal Degree stays in play. Not sure whether the "resolve" wording implies this as well.
I don't think that the Ruined Village option is strictly necessary. Sure, it hedges against the risk of hitting an Action you don't wanna play but isn't that part of the fun of digging for actions via Herald and Golem?
The main change is changing this from a TR to a KC. You get to Throne a card without having to have the card in your hand but the liability is that the other player draws that very card. So basically both players get a card. OK in 2P games, mildly good in 3+P games.
You could of course argue that there is a downside to Regal Degree, namely the risk of hitting an action you do not necessarily wanna play/throne. But I guess that this is more or less compensated via the free-riding this card provides, i.e. you can buy Regal Degrees instead of expensive Action cards.
Regal Decree is a very swingy card that you play at the risk of it being a total dud. This is why I think the reward should be much higher than usual lest it is unplayable. Compare with Tribute which is already a bad card but unless it hits two actions at least it gives you something. Regal Decree literally gives you nothing if it hits the wrong card.
I don't get the comparison with Tribute. Tribute reveals cards and provides vanilla bonuses conditional upon what card types have been revealed whereas Regal Degree digs for an action in an opponent's deck and KCs it. The only risk is that you don't know which card you will hit for but you also have that risk with Golem and I wouldn't label Golem bad.
Sure, normally you throne your good actions. But the large majority of actions are incredibly strong when you KC them. All those terminal silvers now provide +6$ and even a lousy 2$ cantrip with a little bonus like Pearl Diver become double Lost Cities when you KC it (OK, the opponent also draws the Pearl Diver).
So while I agree that Regal Degree is swingy I totally disagree with your assessment that it can be a total dud ... except in Ruins games. And even then it doesn't "literally give you nothing". +3 Cards is worth 4$, +3$ is worth around 5$ and +3 Actions is worth around 2$. You seem to seriously underestimate the power of KC combined with even weak actions.
Why would you want a highly inconsistent Throne Room with only a marginal benefit you can't even influence for the same price as a Throne Room?
The dowside is that you don't know which action you will hit whereas the upside is that you can freeride if an opponent goes for expensive actions.
The only problem I see is that the very presence of Regal Degree disincentives to buy Actions in the first place. But that incentive effect is stronger if Regal Degree is a KC instead of a TR variant.
By the way, I like the card immensly. I just fail to see how a KC variant can cost 4$ or 5$.