First or all, thank YOU for this extremely useful question type! I can't tell you how helpful it is. I just pray it keeps on getting maintained in future Moodle upgrades.
Furthermore, thank you for the kind words regarding the tutorial. It has been up on Youtube since March and I did link to it a couple of month's ago in the comments of the question's page. However, it might benefit from a more visible location.
In terms of changing the label, I might not be the best person to ask and you should certainly get some more feedback, but my own personal opinion is that changing the infinite label is not going to make things too much clearer. In general due to the nature of this question type, its mechanics are not as intuitive as your typical multiple choice or matching question. Any user who wants to use this question type is going to have to play with it to get the idea of how it is supposed to work. Initially, I too could not figure out how the question type was supposed to work until - when all else failed - I actually read the instructions. That and playing with the question type is what got me to understand it and I assume that is what others will need to do as well. Hopefully, the tutorial will makes things a little easier.
In my mind, what would really make things simpler would be that if instead of inputting the coordinates a user could just drag a shape onto the image, stretch it as needed and the coordinates would appear filled in. (The reverse of the way it is now.) However, I assume that is a ton of work if it is at all possible!
Be that as it may, this really is an awesome question type.
Thanks again for all your work!