On a technical level, it is certainly not a mistake to base a new assessment system around the QTI format, information and processing models. As you say, it is pretty complete and flexible.

On the other hand, QTI is neither so amazingly brilliant, nor so widely implemented, that if you ended up going your own way for some reason, no one could tell you that was a mistake either. Ultimately, it is your call.

I'm afraid that don't know enough to help you with the licensing questions.

I do find, coming from an open source background (I work on Moodle) that the culture of IMS is rather alien. When you remember that one of the goals of IMS is to persuade competing commercial suppliers to co-operate on interoperability, some things may possibly be easier to understand.