As a shade tree welder my input would be never put anything more then the bicycles on it.
Your set up is lacking some basic structural fortification and everything rests on the
capabilities of the person that welded it. As the OP mentioned the receiver tube should
distribute the load over a larger area. I can not tell for sure from the picture but the bead
looks a little less then ideal? If you could provide a close up picture of the weld we will
know for sure how safe you are. At minimum take a wire brush to the welds and paint them.
This will prevent rust and possible weld failure.
I am not trying to scare you as it is probably ok for the bikes. I just tend to error on the
side of caution so no one is hurt.