Software-design only with interfaces?

Yes this is good practice. It allows you to focus about the responsibilities of each class without getting concerned with implementation details. It allows you to see the method call stack and as you say gives a high level of testability and maintainability. You're on the right track as far as I see :)

Yes, that is generally a good practice.

I would recommend you to read a good design patterns book, for example this one.

it is targeted for Java developers but I had no trouble understanding all the examples as a C# developer.