I of course think it's possible the this casual acquaintance who walked with her to her car could be involved. But there is so many lose ends and strange stuff going on in her life that weekend and in her friends/SOs actions in the four days after, and so many undertones that she and/or some of her inner circle may have been involved in behavior that increases your risk (i have no idea if the undertones are accurate). It is hard to see that with all that, that this guy who is described as one step above a stranger decided to grab her and harm. I would be really coincidental that on this day(s) in her life so filled with drama crying at work, rampant calls, changing plans at 3:30am when you have work that morning, why are you wanting to do all this if you have to work (are you getting chemical assistance?) she then turns up missing and its due to someone she hardly knows that is totally unrelated to all this. It's not impossible, but it would be a heck of a coincidence ... If I ran things I'd be looking at all the issues going on in her life at the since they didn't seem totally normal, and how that might relate to her being missing.
Ask yourself why hasn't LE gotten a warrant to search this guys house or car what do they know that we don't know?
I think the family thinks what they think because this casual acquaintance being responsible is the alternative the involves the least critical look at their daughter and he closest associates and what they were into and what was going on with her and them that weekend so psychologically it is the easiest option for them to accept.