I don't think that asking Kyubey why a soul gem can only control bodies to 100m is actually a problematic question.
He'll probably give us a non-answer, but it's as safe a question as we can ask.
On the contrary, Kyuubey in PMMM seems quite willing to answer any questions that the girls ask. He answers deceitfully by hiding as much information as he reveals, but he never tells any direct untruths and it almost seems like answering questions truthfully is something that he is
compelled to do. He may not want to answer some of these questions, and he will likely try to hide things, but the following are things that I want to ask him:
1. What are Kyuubey's motivations?
2. What are the "rules" governing his behavior and who enforces them?
3. Why does Kyuubey endeavor to only make statements that are true and not directly lie?
4. What is the mechanism for collecting usable energy from contracts and witchouts?
5. What is the mechanism for converting grief into usable energy?
6. Is the Puella Magi/Witch system something that was artificially created by the incubators, or is it an intrinsically human phenomenon that they merely figured out how to trigger and harvest?
7. Is the soul gem extraction process an intrinsic part of the Puella Magi/Witch system, or was it something that the incubators added separately?
8. If soul gems are artificial, why do incubators create them?
9. Why is the control range from a soul gem to a body limited to 100m?
10. Why does Kyuubey encourage Puella Magi to fight witches (instead of letting them starve)?
11. Why does Kyuubey try to keep his existence and the existence of Puella Magi/Witches secret from the general human population?
12. Can Kyuubey operate multiple bodies at once? Is his race a hivemind?
13. How many incubators/incubator bodies are on Earth? In the galaxy? In the Universe? What about other alien races?
14. How much potential does Sayaka have (compared to an average human contractee).
Also, I don't think we should get too far into the units of our range. Chances are Firn just thought 100m sounded good and it's unrelated to the rest of the magical system. We're unlikely to get data on the underlying structure from it.
100m is from Rebellion where Homura says that the gem control range is 100m, and anything longer requires a witch.