In this video the following sentence is analyzed
それはある女の子が先輩のアパートで行われた飲み会に参加した日
In the video we learn that ある女の子が is the subject of 参加した, while 先輩のアパートで modifies 行われた. Put differently: the outer particle phrase modifies the outer verb, while the inner particle phrase modifies the inner verb.
I totally understand that this is the right way of looking at this sentence (given the meaning of the words, context, etc); however, I am curious if it is logically possible for ある女の子が to be the subject of 行われた and/or for 先輩のアパートで to modify 参加した?
Or are there rules like "the first が phrase must be the subject of the main sentence (not any relative clause found within the sentence)", or other such rules governing what can modify what (outside of pure context)?