Yes, Homura's abilities are limited. I mean, if her powers were limitless, then she could've easily beaten Walpurgisnacht. Also, using magic taints their soul gems, so there's that, too.
Also, people have to consider that Homura has a LOT on her plate she has to deal with. Not only does she have to prevent Madoka from forming a contract with Kyubey, there's also the Walpurgisnacht problem. She needs to make sure Mami and Kyouko doesn't die, and that Sayaka doesn't become a witch. Not only that, but she has to convince them to work together to fight Walpurgisnacht, and even with their help, fighting Walpurgisnacht would still prove difficult. Add the fact that her powers are limited, and that using magic taints their soul gem further, so they also need enough grief seeds for the battle, and they got themselves into hell of a pickle, not to mention that if Kyubey hasn't gotten to Madoka at this point, the Walpurgisnacht situation will convince her to make a contract. All this, while still keeping her mental state in tact, because as Kyubey mentioned, the minute she gives hope she turns into a witch.
As for the whole Kyubey thing, they never really clarify exactly whether he has infinite bodies or limited. If they did imply that he as a limited number of bodies, they never say how many spares he has. He can have a hundred, or he can have a million. Either way, Homura would just be wasting her time, as well as her magic and grief seeds, on something that could take forever and get her nowhere, when she could be solving the whole Walpurgis thing.
Also, just to clarify, Sayaka can heal others, actually.
*Nerd mode activate*
In the spin-off manga, The Different Story, Sayaka uses her healing abilities to help Mami in battle. And yes, it's canon. She doesn't display this in the show because she's working and has no reason to.