Turns out you were right on the money.
Here's an article from October 82 where police specified the man asked for directions to 86th Street in Clive (which turned into a different number in West Des Moines). As you say, a busy commercial street and it makes sense for someone out of town to want to get there.
It's also clear that the police were primarily interested in him to see if he had also spotted the silver Ford Fairmont, which does seem to be the main suspect here. The witness, a neighbor (Smith) who heard the slamming door and saw the car speed away, without any sign of Johnny, is the only one to have seen the actual abduction site.
The account of a "tall man" emerging from between two houses and walking towards Johnny is more elusive;
here is an article from November 82 where the Gosches' PI Whelan mentions him. Further on, in March of 83 it is
implied that the witness who saw the "tall man" was not Rossi (the adult who was at the dropsite at Ashworth and talked to Blue Car Guy at Johnny's request), nor brothers B (later referenced as separate witnesses seeing Johnny at Marcourt on their way back from Ashworth). That leaves paperboy Mike S (who saw Johnny talk to Blue Car Guy at Ashworth) but that doesn't seem probable as his route didn't take him up to Marcourt. I honestly suspect it is a mix-up starting with
this article from two days after the abduction - where a paperboy (almost certainly Mike S) says he saw Johnny talk to a man at 42nd and Marcourt. This seems to be a distorted version of his documented sighting of Johnny talking to Blue Car Guy at Ashworth (complete with police sketch), and may have taken on its own life from there. Either way, I can't find much else on him.
Later testimonies, all filtered through Noreen Gosch and her PIs, add a bunch of stuff to this, none of it reliable. The paperboys statement that Johnny thought Blue Car Guy was "weird" (attested in 82) evolves to him being "scared" and "going home". Brothers B saying they saw Johnny sitting in his cart at Marcourt (presumably to sort his papers) evolves to "sitting slumped over" implying something was off. And poor neighbor Smith, whose simple testimony of seeing a silver car drive away evolves to him seeing the same blue car from Ashworth (1985) and then seeing the entire abduction complete with tazer (2005). Much of this is then repeated as fact in media and articles discussing the case. Chris Birge, who lived in the neighborhood, says Smith denied saying the stuff Noreen attributes to him.
It's easy to see why all the focus landed on the blue car. It had police sketches, it was constantly referenced in newspapers, and after a while you get the impression that Noreen's PIs wanted it to be the abducting car. All other strange sightings (woman taking photographs in the neighborhood, etc) are later and comes from Noreen's PI's investigations.
Of the later developments, the only one I give credence to is that of
"Yellow Bag" who worked as a paper boy at the time. His claim that a man in a white Ford Fairmont tried to get him into his car the winter before Johnny's abduction, and that he later saw that man with an employee at the newspaper, Millhouse (a known pedophile).
In my view the likeliest scenario becomes this:
* Johnny leaves home with his cart, goes to pick up papers at Ashworth.
* Blue Car Guy is lost trying to get to 86th Street in Clive, stays near a paper drop at 42nd and Ashworth to find someone awake to ask. Mike S, another paperboy, sees this.
* Blue Car Guy asks Johnny for directions to 86th, then drives off, though whatever Johnny says isn't useful.
* Blue Car Guy returns to the dropsite, asks Johnny again, now in a rush and agitated. Johnny calls for Rossi, an adult there to pick up papers for his kid, to help him. Rossi gives the man directions, after which Blue Car Guy speeds off and takes a left on 39th, trying to get north to Clive.
* Johnny walks his cart up to 42nd and Marcourt where he preps his papers for delivery. Brothers B, who are delivering further north, walks past Johnny while he's working.
* As Johnny is prepping papers, a silver Ford Fairmont drives up next to him. Whatever happens, it ends with Johnny going into the car (perhaps at gunpoint, or dragged by his bag), and the car speeding off.
* An hour later, the disappearance is noticed.