I’ve got a parent plug-in which holds generic code for all my plugins. Instead of adding this code to each plugin (which is a maintenance nightmare), i want to seperate this.

My questions:

1) Is this allowed?
2) What is the preferred way to build a dependency into your plugin?
3) If it is not allowed, any tips on how to avoid the maintenance issues?

You can declare all class in parent plugin and check it in child plugin if class exist on parent plugin then child plugin activate else not working.

No.

A plugin has to work and do SOMETHING on its own. You’ve described a library, which is not permitted.

I recommend a tool like gulp or grunt to aid your development process when using libraries.