You can use a bit of a stick approach in combination with the Custom Wizard Plugin
With this you can:
Now that would definitely help many forums. 
First you want to increase viewership
You can do that by sockpuppeting for 2 hours a day for 4 months, and making your posts fun. I did that and now I dont even need to login, it’s got it’s own self-sustaining community now
If that doesn’t work then there’ll be no way to get it to work sans paying people to be on it
probably a way to automate sockpuppeting to a certain extent
We had users move from one forum to ours as the first forum was “retiring.” I had posted a globally pinned topic on the old forum about a month before it shut down giving them the opportunity to remain in contact with others by joining our new forum. I set up a separate mailbox and posted it on the old forum so they could let us know they wanted to join. (I did tell them it was a “private, by invitation only” forum.) These people took the time to send an email expressing the wish to join… as well as making some other well-wishing remarks. Once they joined, they only logged in once in a while - just as they did on the old forum. So we decided that this is what they wanted. I did think about sending an email to them just to let them know that we’ve missed them, and maybe let them know how long it’s been since they logged in. Before I even had the chance to send said email, about 5 of them logged in. Two have been logging in at least once a week now… a big difference from an absence of over 4+ months.
I’m still thinking of using a personal reminder email as mentioned above. In it I can let them know the topics everyone’s been discussing and maybe even who joined since their last login. Who knows? Maybe they’ll recognize a username and decided to pop back in.
In your case, maybe renaming your general category to “General” or “Random Thoughts” - depending upon what topics you have in it, and renaming the “Latest” category to “Around the Wheel” or “Current Spokes” (pun intended).
Another way to help push someone into posting is by asking questions. If someone planned on just reading, sometimes they’ll say “I know about that” and will reply. Once they’ve ‘broken the ice’ they’re more apt to post again and again.
You could also let them know that by posting, others will post also. Sometimes all it takes is for someone to see others posting and having a conversation. Who really likes just to sit by the wayside, listen and not want to get in on the conversation. And where you’re trying to get people to sign up for your training, something as simple as changing the word “Discuss” to “Learn more” or a combination of both may help a little.
As to “Your first notification!” popup… the vast majority of people automatically close popups (thanks to all those annoying popups on some sites wanting you to do this and that or they want to send you messages). There is nothing more annoying than trying to read something and having a popup gray everything out so you have to stop reading. People inherently don’t like “interruptions.”
As an aside, you could make visiting your preferred topic a requirement before continuing to the next part of the course. In other words, make participation a part of the course. It would be up to you whether or not they’re required to post or reply in that topic.