What we know is that he didn't come forward to eliminate himself from being a suspect. Why not? Now as the cctv was likely known not longer after the disappearance, it's possible he was identified from that or with some other evidence. The police likely monitored him after that to see if he would lead them to her, if they arrested him early on and he had her locked her up some place it would endanger her from passing due to lack of water and food etc. but as he didn't, they then proceeded to arrest him because at this point they do not believe he is holding her.
The fastest way to then hide a body and make it look accidental is the river which was right there anyway. It can also conceal injuries sustained. The police are just going through the motions and putting on a display of positive action for the public and family. I've no doubt the police are almost certain she will not be found alive and the body is very likely in the river.
I don't know if he will be charged as you can't disprove he didn't pick her up, saying he was helping and she came onto him, he rejected her and she left walking into the night. The fact it is so close to their time being up to charge him doesn't mean a charge is likely, it means they have little concrete evidence and he could be released 10 minutes before the deadline.