King Tauri, is, in fact, a cunning guy (atleast when he needs to be)
Spoilers for all the 3 books.
Making all the heirs to the throne go to the infantry isn't just out of concern for their lives, but also to ensure that the dragons do not have power over the kingdom.
How? The elder of the six dens have power over the other dragons. On top of that, their riders have strong signets. They will automatically be essential to win wars.
The elders were against anyone doing anything, so naturally their riders, who automatically rise higher up the ranks (nepotism or maybe not) have no choice but to obey.
Their humans, in turn, protect the King, and the dragons don't really have much power over the humans at the end. The dragons who do not obey, their riders are killed because they are threats and can defy the King once they rise up the ranks.
Another smart thing Tauri did was not stop the riders from going to Tyrrendor. He knew that one day they will come back, they had to, because the Vale and the dragons' connection to that was a big liability.
Him sending General Melgren and General Sorrengail to ask for help was to show the direness of the situation. Also, dare I say, a way to guilt trip dragons into coming back.
He sent Halden with the squad not for supervision but to make sure the alliances fail. Then, the only thing Tyrrendor and the fliers would have left would be Navarre and its resources.
He then is against the taking in of refugees because it's Xaden on the throne. Xaden has a dragon which is unpredictable. Also, he is a marked one. On top of that, Violet is in a relationship with him. And Violet Sorrengail being on their side guarantees their win. Also, Tyrrendor has a large army, which itself constitutes most of Navarre's. It can become it's own country.
Lilith Sorrengail was an insanely smart woman to not go against Tauri. For whatever period of time, it ensured that her children were safe. On top of that, even if she died, her children had no one who wild take them in. No offense, but Xaden had Lewellen.
The battle of wits will not be between Violet and the Venin. It will be between Violet and Tauri.
With Xaden gone, Violet better get her shit together, or Tauri will win even before shits starts to go down the hill.