You should have bought the entire journey (Olten-Modena, or Olten-Milan) as one through-ticket, only then the seats would have been reserved from Olten. As Olten-Domodossola is part of the Swiss rail network and therefore considered a national trip, you cannot reserve the seats. It is not possible to reserve seats on Eurocity trains for national travel.
As a general saying, in Switzerland, there is free seating for non-reserved seats, it’s first come, first served. I suggest going to the coach in which you have reserved your seats from Domodossola, they might be already free from Olten, or they will be likely free after Bern, where many people usually get off.
A change from platform 11 to 12 in Olten is easily possible. There is a stair free underpass in sector B.