Was there ever any kind of timeline confirmed as to when Suzie last saw Sherill prior to the parties? The sighting at the restaurant is pretty specific, and I think it's possible that the three women went there sometime that evening - it actually sounds reasonable that Sherill would take the girls for dinner after the graduation. If Suzie was upset that day (as is often reported), taking her to a familiar, friendly restaurant would seem like a good idea to raise her spirits.
As for the van sighting - is it completely incomprehensible to believe the perp forced Suzie to drive? The whole speech that was heard does sound completely cliche for what the average person would believe a hostage/kidnap situation to be, but it's hard to completely discredit it either way. One abductor may have wanted to subdue the two other women, but more than one perp and there's no way any of them were going to be put behind the wheel. I have no idea how those vans were designed, either - especially in terms of the divide between the driver/passenger seats and the back portion. It's very likely the sightings are genuine, but perhaps with a little fiction added into the mix. One thing we can be sure of is that the van would've attracted little attention at that hour on a Sunday morning - even in the summer.
