Pushing the piston should not damage the ABS. Which brake pads did you replace? For the front, C-clamp is used to push the piston but for the rear, you need a brake piston adapter to TURN the piston in. Are you sure you did not 'touch' the ABS wheel sensor?
If you have a discount for ABS as part of your insurance, then it's not a good idea to disable it as you may have a problem with your provider in case of an accident.
The best way to identify the affected component is to use the Built-in Diagnostic. All you need is a jumper wire connected to Pin 8 of DLC (Data Link Connector)and chassis ground. Just follow the instructions on pages BR-48 and BR-49. Once you know the fault code, go to page BR-61 to identify the defective component and how to fix it. Refer to the link below. It's for the 2001 I30 but should also apply to your car.
http://www.nicoclub.com/FSM/i30/2001/br.pdf