Yes, I understand. I reckon you don't recommend developers that could do the job?

Nope.

But maybe it would be a good idea if we'd booked a few consultancy hours from your webpage, we could discuss things in detail via Skype or so and I could take notes etc. I guess that would make the work much easier for me. What do you think?

I've basically already provided you with consulting here at no additional cost.

The most complicated aspect of all of this is becoming familiar with CB Conditional which the below videos should help get you started.

www.youtube.com/playlist?list=PLp0puRITgC7PxD6MvmDwNNIi2HNcYxs7u

For usergroup assignment using CB Auto Actions you'd use the below initial configuration with any action of your choosing (e.g. in this case the Usergroup action).

Plan Active
Global
Triggers: onCPayUserStateChange
User: Automatic
Access: Everybody
Conditions
1: [var3] Equal To PLAN_ID_HERE
2: [var2] Equal To A

Plan Expired
Global
Triggers: onCPayUserStateChange
User: Automatic
Access: Everybody
Conditions
1: [var3] Equal To PLAN_ID_HERE
2: [var2] Not Equal To A

The below videos will also help get you started with CB Auto Actions.

www.youtube.com/playlist?list=PLp0puRITgC7McQcw0Sli1qfSUQGwmflzi

It's honestly significantly less complicated than digging through source code, extracting chunks of code, and modifying them your needs. Just takes some time to become familiar with the plugins.

What is actually the difference to

That loads the plugin class file. It doesn't load plugins. The above call actually loads plugins. cbimport is basically going away at some point so for ease of migration later I suggest just using the below to load APIs after including the foundation file.