Pretty much agree. 2 full days in MVD is "plenty" on a limited time trip. The Colonia stop is worthwhile, but a full day there is more than enough. What I do not know is what sort of luggage storage facilities there may be during your Colonia stop. I assume that Buquebus or Colonia Express has some sort of service (or that a service is available at the terminal).

Day trip to Cordoba - don't. Not dissing Cordoba (I've never been except for a flight diversion, sitting on a plane for 2 hours), but you'll be turning your trip into an If it's Tuesday, this must be Belgium experience. If you feel the need to get out of the "Big City" for part of a day, might I suggest taking the bus to La Plata for some interesting architecture. (I won't suggest taking the train to Tigre, although others may. The only reason I'll return to Tigre is if I want to do some furniture shopping.)

The "worst" that can happen when you spend not enough time in a place is the desire to return - as opposed to spending too much time somewhere, wishing you had allocated your time differently. If you spend one more day @ MVD and one less @ BUE, you'll probably feel the latter. (Not dissing MVD either)