They weren't certain he was the suspect, hence why they were talking to him.

There are more options than just arrest someone or don't talk to them at all. It looks like the police were trying to ascertain his identity in a peaceful way as he matched the description of a suspect in that area (How valid this is we can't know yet). It was the suspect who became violent and escalated the situation

Again we can't see from the video what led up to him being in his gateway but I'm assuming from his earlier behaviour he pushed past the officers and they had to forcibly bring him back out as at this point they still think he might be the suspect they are after and his behaviour is only reinforcing that suspicion.

And if they weren't really sure he was the suspect and would have just let him go if he'd given his name, then why were they wasting time hassling him, and did they just feel the need to put him in his place for being uncooperative.

Probably not straight away if he'd just given a name, but as soon as they could be reasonably certain of who he was then there is no reason why they would keep him there any longer. Unless of course he becomes violent and obstructive towards them.