@stephane-maarek this is very interesting! Since you are a Mentor, have you taken Claire’s Online Community Leadership course? It addresses some of these issues.
I think you need to look into why people take courses, to begin with. Are they trying to get a certificate so they can include it in their resume? Or, are they looking to self improve? Or are they taking a class just for the fun of it?. This influences completion more than anything else.
I would audit classes to learn something new or just for the fun of it. I don’t care about completion or a certificate. I could pick and choose the topics I want to know about. I enjoy thoughtful discussions and usually am an avid participant. There is no doubt that learning how to positively interact will make a difference as to whether or not someone responds. I view discussions as conversations. When someone tells me something I question it, ask for further clarification, just positively comment or disagree and want further discussion. As a mentor and leader, it’s important to react to people, make them feel comfortable, and inspire further thought.