The comparison with witch bombing Mami is kinda 1:1 - the reason it hits Mami so hard is that she's been mentoring and helping younger magical girls, and now realizes that encouraging them to contract was leading them to this awful outcome, that her efforts weren't just in vain but were maybe actively harmful, and that she might have slipped too far down the slope to stop that or recover. (Which is why she tries to halt the process as directly as she can, in that one scene, to my read? It's the only thing to stop it getting worse that she's thinking of.)
Homura is similar - the potential bomb means that not only has she been struggling without success for a long time, but that the struggle itself has been making it worse, that if she had given up trying to save Madoka it's possible Madoka would have been safer and better off. It invalidates what is essentially her reason for living, right now, and if we aren't prepared to immediately get her across that gap and realize there's still a way to succeed that's an enormous blow. It's a much more personal fear to touch on for Homura than Mami, who we've spent more time helping and building up structure so she has hope and feels like she's making a positive impact on things - Homura is still basically betting on the plan with us this loop to get there, but what does she have outside of saving Madoka to think about? So that's where the risk comes in.
Ok, I think I get it now. I still believe it's best to tell her, but it might be better to set the sage a little.
The trick to get her over this is two fold. One, we redefine Homura's definition of failure. Because she hasn't failed, not once. She's saved Madoka, Sayaka, Kyokuo, Mami, and every other girl who would have witch out during her time loops. It is a fact that without her efforts, dozens of of people would be dead or worse. And she's been getting closer and closer to her goal. It just takes time.
The second is is connecting saving the world to saving Madoka. Which is true because Madoka won't accept a world where she has to live with the Maguda system and the Incubators in general. Regardless of wheather or not Homura kills Wally. Because Wally isn't really the problem for her. She's just the catalyst that the Incubator uses to show Madoka that she's needed. And it has a multitude of ways of showing that.
I've been thinking that we can take care of all of this in one go, by connecting Madoka's potential with our ability to help things this time, and how Homura's action's have saved Mami and created us. By convincing her that she has been inadvertently doing an excellent job of saving the world, and by extension saving Madoka. I still think it's possible, but it might pay more to set some ground work for this. Like, we don't need to bring up her potential to get her to see that she hasn't been failing. We can tackle that first, and then try to connect saving Madoka with saving the world then connect her potential pumping with her putting her self into a position to accomplish the final win.
we can talk this out with Mami, including our worries about her rejecting her wish and if it might be worth it. But the way I see it, Homura's wish is perfect for what she set out to accomplish. Maybe not in the way she was expecting, but perfect in a way that is more real then most wishes turn out. We just need to convince her of that.
Edit: one caveat, the part about not failing can't come from us. We are here to help, literally born for that in our eyes, so we would tell her this regardless. That's just who we are and it might be why we can't fix this on our own. But it can come from, and mean so much more coming from Mami. Who owes our existence to Homura, and every drop of hope that we have inspired in her.
And that actually might be something that Homura can
understand. The feeling of having someone come in like a whirlwind and change your view on this earth forever. What might it mean to Homura, to know that she was responsible for bringing someone else's
Madoka. And not just anyone, but
Mami, her mentor. I have a hard time believing she would reject her wish if she understands that it brought something like that into the world.
