Paper accepted at a probably predatory conference, how shall I proceed?

Don't go.

You write that it's a valuable experience to attend a non-undergraduate conference, and it would indeed be the case, but you won't be attending anything like a non-undergraduate conference. Example story, and that actually reads like one of the "better" predatory conferences, since I've read about other predatory conferences where keynote speakers don't show up and attendees talk about completely unrelated stuff (e.g. a number theory talk at a food science conference).

This kind of event won't help your CV either, because if someone checks the conference out they'll discover that there's no quality control, meaning the quality of whatever you presented is now suspect.

Since you gain nothing by going, is there still worth going? Possibly, but any reason for going would be much less tied to the conference than to other things, e.g. perhaps you know someone who lives near the conference venue and have been looking for a reason to visit. If you don't have these reasons, you might as well save on the cost of travel.

The first step is to get a professor in the area of your research at your university to help you. This does sound like a scam conference. It sounds as if you found the conference yourself, without guidance.

The tougher issue is the paper withdrawal. It probably is worth some effort to do this. If what you sent in is weak, having it out there might embarrass you later. If what you sent is was strong, publishing in this venue might block you from publishing in a better outlet or presenting at a good conference.

At most legitimate conferences, writing to say you cannot attend (didn't your cousin just moved her wedding day) will stop the publication. If you have not send them any money yet, perhaps withholding money will stop them publishing.

This is not something older professors dealt with during undergrad years. Browse this site and you will see that you are not the only one caught in this sort of scam. I like to think graduate admissions committees are getting used to seeing situations like this.