If a card in your hand has {X} in its mana cost, X is 0 for the purpose of determining its mana value.

The value of X is calculated only once, as Kinscaer Sentry's last ability resolves.

You choose the player, planeswalker, or battle the creature you put onto the battlefield is attacking. It doesn't have to attack the same player, planeswalker, or battle as Kinscaer Sentry.

Although the creatures put onto the battlefield with Kinscaer Sentry enter as attacking creatures, they were never declared as attacking creatures. Abilities that trigger whenever a creature attacks won't trigger when the creatures enter attacking.