Hi @drmonalisa and thanks for your query. So long as your intentions for development are for interaction only, and you are not going to store the gaze data for long term use, then you do not need to worry about purchasing an analytical use license.
Certainly, your project intentions sound very interesting! In fact, we are aware of a program that provides gaze aware zones called Project IRIS (http://iris.xcessity.at/) the development has been halted for a while, but It may still suit your needs?
In terms of sharing your code, again so long as the intention is not for analysis or storage, then you are permitted to share your code freely online including the Tobii DLL as appropriate.
Please do let us know if we can provide any further information.