I thought the York coppers had always assumed she'd been taken on the way to work - it was said at the time, her chef's whites and mobile phone and backpack were missing, which implied she vanished on her way to work. Also, one of their suspects was a man who worked on campus who had sometimes given her lifts to work, (presumably at that early hour, too). The fact he was questioned means they were always looking at people who might have abducted her on the Thurs morning?
As for the couple seen arguing - hasn't that since been debunked, as the woman had blonde hair (like in all the missing posters) yet she had brunette hair at the time she disappeared..? If it's the people arguing near the park gate, that's about a good 5 - 10 min walk from her house and not on her street or the street with The Nag's Head on. But a way round the corner - would be not the most obvious place to pick her up if it was someone who's previously given her lifts - surely they'd stop on her street (and they didn't, as one of those new houses opposite had a camera, IIRC?) or outside the pub or where the shops are before the turn up towards the park, as there's a carpark at the shops there. Also, not a great place to publicly argue with a person you're about to kill as it's overlooked by houses and people park on that street so anyone else about to start an early shift, would be out and about, where those gates are.
FWIW, I think the police have a suspect - possibly the ex boyfriend they knew about. (He was alibi'd out by the wife, I think?) Sounds from that like they didn't have enough for the CPS?