Because they did not know what the outcome would be of sealing all 52. Besides, they also did not know the cards would be permanently locked either.

I still say the plan to unseal the Human Undead would have been a solid idea

Which would mean completely destroying Hajime's personality.

But Hajime knew, why didn't he tell them? Why didn't they just unseal Shima and Category Queen before Tachibana sealed the Category King and avoid any potential disaster if Joker won? It's a minor complaint, and besides the ending was perfect with Kenzaki's decision.

I don't think Hajime knew exactly what would happen. And Kenzaki had total faith in his friend that he wouldn't destroy the world - nobody was really prepared for Dark Roache swarms.

This is probably the biggest reason why they didn't make any 'preparations'. They were told Joker would destroy the world, so they probably expected Hajime to go crazy to go on a massive rampage. They weren't expecting Joker's win to herald a swarm of ultra aggressive infinitely spawning roach monsters.

I know it's a minor complaint, just nitpicking. The series was really good, and the finale was excellent.