my only complaint is that we didn't see the prince and the saintess actually get any sever...

My complaint is that the king is like what, 20 years older? And he's marrying a teenager. But hey, what should I expect from a manga like this ╮(￣▽￣)╭

I mean you could look at it that way but he looks too young to be in his 40s. I am thinking there is an age gap between the Uncle and his brother. Say for example they had their second child when the first born was 9. Another example is a 19yo had a child and then her parents recently had another child. Age gaps, but I am not convinced the Uncle is old maybe between 26-30.

I thought that the uncle must be more than 30. If the crown prince was like three when his parents died, the uncle couldn't be a kid to become a stand-in king...
Which also leads me to another problem - if the younger nephew will probably become a new crown prince, what about the kids uncle had with MC? Aren't they a danger to nephew's claim to the throne? But that's too much thinking for a one shot, I know

Why not? There are many series where a 6-18 year old took the king's position. Its not unheard of in manga or even history.
As for the second part, I wouldnt think too hard on it as it ended before we found out whether the ex-crown prince did prove himself. From the story though he seemed to be lacking, unable to grasp the situation or believed his lover quickly than conduct an investigation. So he probably dropped out and lived happily with his lying affair partner while the Uncle and his queen ruled.

Ah, so I re-read it and there is a scene sharing how the previous king and queen died. The previous Kings children were 3 and a baby. The Uncle is shown as a child around 6-8. So not a huge age gap. He isnt in his 40s but around 23-25yo, it was just too brief we missed it.

Ooooh, okay, I see, I must have missed it too, or thought that the kid uncle is crown prince shown a bit ages up, because after that they were showing how MC and prince got engaged...