Bob,
Given the umpires explanation, would you not agree that he misapplied the rule about the ball getting by the fielder, regardless if he actually touched the ball at all and that the interference call is protestable? The umpire didn't state there was another fielder who could have made a play. His argument was that because the fielder never touched the ball the first time, he still had a chance. Only the first chance by a fielder matters. Once it gets by, only a second fielder with a chance factors into the situation.
FED 8-4-2k

k. is contacted by a fair batted ball before it touches an infielder, or after it passes any infielder, except the pitcher, and the umpire is convinced that another infielder has a play (5-1-1f, 6-1-5).

The text in red doesn't apply. Text in blue applies, but the text in lime doesn't. So, the umpire has misapplied the rules, has he not? I would protest the interference also, because of the misapplication of 8-4-2k.