While Dancing and Singing, the character's evasion is lowered to 0, like any other charge. While charging, Blade Grasp and other Brave based attacks do not work. I have not tested it with Sunken State specifically (to the point that I can remember it clearly), but it's reasonable to assume that the same applies to it. If you have not actually performed the action you said with those Dancers, it's more reasonable to believe that there would be no reaction.
Lancer's Jump ability will do 3/2 damage as calculated by (PA*Weapon Power) if equipped with a spear. Thus, a Lancer that has 16 PA, with a Javalin of 30 power, will do 640 a jump. Attack Up is calculated at the end, with the damage. That would mean it's a 960 damage with each jump, not 640. However:
Lancers and Knights both have the same class speed bonus. However, Knights will get their turn before the Lancers can come down from a Jump. This gives some opportunity to:
Put up Defense Up, and reduce the damage by 2/3. That only requires one knight, anywhere.
You are assuming that the Lancers get a chance to jump, and it's probable that the Knights can't get right next to the Lancers right away to attack. Thus, they will only move.
When that happens, the Knights do not need to charge up all 100 CT to act. It starts at either 33 or 50. Assuming the lower, 33, they will have to charge 166 CT at normal speed. With the 3/2 speed bonus, however, it's only 110 CT to be charged.
Your Lancers:
66 CT to be charged before getting a turn to act. They, you have 50 more to charge for the Jump ability. That's 116 CT to be charged. It's more than enough for the Knights to move out of the way.
With Setiemson, the Lancers will not have any back EVA, givin the Knight a simple option of just attacking from the back.

By the way, you've yet to come up with a reasonable way to beat my Wizards with any of the current party without totally ignoring a huge number of factors.