On the other hand I heard rumors that an ingame trading mechanism exists in multiplayer without throwing items around.
yes trading does exist but it is a bit buggy at times (i.e. items disappear from your inventory and do not appear in the other player's etc.) thus dropping items is a quite common way for me at least to give items to other players. The safest way to trade atm is to put items in a chest and let the other player pick them up.
I get that you want to make it as simple as possible for everyone but sometimes the simplest is not the best solution as you said. I am curious now what your original thought was and if you could just ask red51 for the methods to make it happen (in the API requests section). think about it.