I know this is years after the fact, but for anyone here in the future, you can just implement the messages via the label and dialogue system rather than placing them in the messages section of the script and trying to pop out the ones you don't want, this allows for modular dialogue to play out, which I think is what the asker was going for here. For example I'm using a hacked together copy of the message system to pop up a fake text screen in the center of the screen, and implement the messages like this :
$ girl3_message = girl3_message + [messageitem("Hey new guy, asked Liv to grab your number for me.", False, 0)]
$ girl3_msg_count += 1
show screen chat_message(count=girl3_msg_count, girl=3, messageholder=girl3_message, gname=girl3_name)
pause
$ girl3_message = girl3_message + [messageitem("Mind coming to meet me out in the woods? Got a surprise for you.", False, 0)]
$ girl3_msg_count += 1
show screen chat_message(count=girl3_msg_count, girl=3, messageholder=girl3_message, gname=girl3_name)
pause
Adding new messages in that method makes filling out the array rollback safe, and the message isn't added to the chat system until that section of the label is reached, so if you talk to her about something else sooner, or choose to not have this conversation, it won't go into the chat history.