Why didn't Kyuubey just send Homura back in time once rather than giving her the power of time travel?
Because Homu a shit and Kyuubey knew she'd fuck it up multiple times.
Because you didn't watch the show.
That's not how it works. Kyubey is just a catalyst; the girl's karmic potential is what makes the wish possible.
Kyubey even specifically says "your wish has prevailed over entropy" when Homura makes her contract. If the Incubators could do that on their own, they wouldn't have created the magical girl system in the first place.
If you don't understand the connection between entropy and time travel, you can start with