Okay, first of all, I'm sorry for the late reply. It's been a week, but I've been able to test what you said. And yeah, by lowering the text speed, relative pauses work.
What you suggested last week is good. Here are my thoughts/suggestions:
1. Please add a comment that warns about this to the .rpy file. As far as I know, this was only implied in the explanations within the file. So it would be useful to see this being given a warning.
2. As you said, you can implement a message that appears under the "Sentences" bar when the Text Speed is maxed out, warning that the text speed shouldn't be maxed out if the user uses relative pauses.
3. You can possibly make it so that the text speed can't be maxed out when the user is using relative pauses. I don't know if that's possible. For example, maybe you can make it so that the max of the text speed is 0.1 instead of just 0.
So yeah, feel free to share your thoughts on what I suggested. It's okay if your reply is a bit late. After all, I haven't responded to you in a week.