That's still an incredible amount of assuming. I mean, a massively incredible amount of assuming. If Mutt ran into the Dean (who was the only one besides the KGB who knew where Indy was going.) then obviously A) It would have had to of been before Indy left for the station at Indy's house but after the conversation we see (which we know didn't happen.) or B) he ran into him later at some point after he left Indy's house and since Indy was packing we knew he was leaving very soon so the chances of him meeting one of the two people he told where he was going before Indy boarding the train are extremely low. Normally, I'm all for the introduction of new characters and they all pop out of nowhere, however its the introduction that you have to figure out or else it'll just seem like "OH LOOK, KOEPP CAN'T FIGURE OUT HOW TO ADVANCE THE PLOT, QUICK JUST THROW THE SON THERE!" It'd have felt not so forced if Mutt showed up at Indy's house just when Indy was about to leave or something, as it is in the film Mutt just seems to come from nowhere, no idea where Indy was, no idea really what he even looked like.. yet somehow he knew exactly where to be at the exact time and knew the exact person to yell for. I don't know, maybe I'm thinking too hard here.