The shortest itinerary includes only 5 minutes to change trains in Poznan. Is this enough time or should I a choose an itinerary with a longer time in Poznan?
In my opinion you have less than 50% chance to make it, especially with an old lady. Try to ask train staff what are your chances. They usually know things like if they will be on time and if the other train is waiting and on wich platform.
PS. But be carefull what kind of ticket you have, there are about tree different rail companies on this route. If you will have wrong ticket, you will have to buy another one (you will can get part of the money back on the station though why bother with that..). So, about an hour before arrival time to Poznan, with your ticket in hand and maybe along with some good soul speaking both polish and english go to the conductor and ask him what are your options.