I think it depends on how active your 8 month old is. If they are very active they will not want to stay in the bassinet. OTOH, if they are to the type to sleep on planes and long car rides then you may be better off with the bassinet.

You mention that you only bought three seats and then say that you might get four together. How are you going to do this? Are you talking about buying another seat, or are you hoping to snag an empty one? On international flights I have never known an airline to give a free empty seat to an infant as they might on a domestic flight. You will have to pay 10% of the applicable fare for an lap child and usu. around 75% for a INS in a seat (sometimes as high as 90%).

For the two year old bring a variety of activities that they like to do, and low sugar/carb snacks.

Have a safe trip!