According to nbc sports web site, coverage of the Derby starts at 4pm EDT. And the MLS game starts at 430pm EDT. So I'd say that from 4-430pm, coverage may shift to NBC Sports Network and only if it goes past 430pm, you may have a problem.
Since you are in Chicago, I think your gameplan works. I have a feeling that in NYC metro and DC, they'll keep the nhl hockey game on the main NBC until game's over. I know people in this area bet on the KY derby a lot, but there's a level of excitement today in the office about the rangers game last night that, I am pretty sure, even a casual fan might be glued to TV on Saturday afternoon, if not for the entire game, definitvely from 3rd period on and I doubt NBC in NYC would move the rangers game away from channel 4 if game goes to lots of OT like last night.