55k rounds is a pretty decent round count for any gun. Yes there are some that will last longer, I've got a .40 S&W Beretta with over 150k through it, a Shadow with 72k+ through it and another with around 45k through, all 3 guns are still in excellent shape. But I've also toasted two Berettas, one at 11k and the other at 22k, I've got a Shadow with a flaw in the frame that I had to repair with steel epoxy, and I've seen Glocks break locking blocks and slide rails at under 5k. Each gun is like a lottery ticket, but hopefully with better odds.
Now as for your gun, honestly, unless you intend to try and shoot another 55k through it, I wouldn't even worry about those cracks. Smooth em out if they are causing binding issues with the gun, and use it as a plinker. Nothing dangerous is going to happen, even once they become terminal. If CZ is willing to replace the frame, awesome, but 55k, on a rental gun? I wouldn't expect them to, in fact I'd think it ludicrous if they did. Even better, buy a .22 Kadet conversion for it and throw the slide on a new frame. Double win.