Why would supposedly Genre Savvy Columbus get the drop on him twice? He knows all about zombies, not people.
Why does Wichita do something as rock stupid as turn on all the lights and music in an amusement park, when you think she'd know that that will just attract every zombie within miles? Is she trying to get herself killed? Yes, she is. They've come all this way, betrayed and abandoned so much just to make it this far, and Pacific Playland isn't really a survivors' colony but just another abandoned building. There's nothing left to fight for or hope for that she can see, so all they can do is eat, drink, and ride the merry-go-round, for that night she and her sister will die.
Having done quite a bit of acting in her con days, Wichita could have been acting scared to avoid cluing Little Rock in on her Death Seeker tendencies. Outright letting her little sister know would just make things worse, as Little Rock might not share the same desire for death as her older, "wiser" sister.
Alternatively, Pacific Playland is to her what Twinkies are to Tallahassee - the last reminder of her old life she has left. And her only weakness. She just didn't think her actions through because she just wanted one last taste of her old life.
How would someone like Tallahassee know whether Pacific Playland "totally blows"? He's been there, with his son. And thanks to that, he's got a game plan for zombie-killin' there at the end of the movie, and even knows where the Twinkies are kept.
In Columbus' flashback, he is shown playing World of Warcraft. Close examination shows that he played as a Night Elf, a race belonging to the Alliance. Apparantly Columbus has been battling the Undead long before they came to his doorstep.
The sisters' first con job plan on Columbus and Tallahassee seemed to hinge on both of them being unable to bring themselves to shoot Little Rock. In hindsight, a strong note of panic can be heard from Wichita when she implores Tallahassee to wait when he aims his gun at her sister - it seems that their plan came dangerously close to going horribly wrong. If Tallahassee were just a bit less compassionate, he might not have hesitated and instead blown Little Rock's brains out before Wichita could stop him.
It is amazingly stupid. You'd bet 9/10 zombie apocalypse survivors worth their salt - people like Tallahassee and Columbus who survived on their wits (with a dash of Genre Savvy) not by their luck - would not hesitate for a second to blow a potential zombies head clean off.
It's extremely scary to think that Wichita was a second too late or Tallahassee a second quicker Little Rock's brains would be everywhere. Or if she was conning the wrong people. Was it just luck? Or maybe Wichita purposely didn't care wether Little Rock was killed or not, not because she's evil but so that her sister wouldn't have to get eaten by a zombie or be left alone to fend for herself if Wichita got eaten.