You don't admit to a crime in interview and then get bailed "pending further investigation"
You might if they are hoping to find evidence of additional charges, or more serious charges. But the fly posting is bang to rights, she was caught in the act, and they've got CCTV of her doing it on other occassions according to the police, and they found similar stickers at her home and she admitted it she claimed in the Savage Minds interview. The one thing they are not short of is evidence.
Whether they can make the public order charge stick in addition to criminal damage is what's really in question - and that will be down to whether CPS lawyers think that what it said on the posters she put up has a good chance of being found in court to have caused alarm, harassment or distress. Given we don't actually know what it said on all the stickers and posters she put up then that's difficult to call. One option would be to drop the public order charge and go with criminal damage and ask for it to be aggravated to a hate crime, in which case it will be down to the magistrate whether she should receive an aggravated sentence.
The other is deal with it with a penalty notice/caution, presuming she is prepared to accept that. But I don't really see a world where it's dropped, what does that say to the community of Newport, people can come and deface public property under the police's noses with no consequence? I doubt that will go down well with either the council or local business community, lots of people really don't like illegal flyposting.