I am not a huge fan of their use myself, or really life debts in general that are enforced. But in this case I think its somewhat justified. But you are totally right, Elesa if she takes him with her has to deal with Shinji and his wonderful nature and she not going to fully enjoy that. She does however needs something to prove that Shinji is not just blowing smoke at her and can man up to what he is saying.
Another way to look at this choice to me is not the whole what option gives Shinji (us) the best possible outcome with the least number of risks, but to think of this as Shinji paying his debt to her. If their is one thing I think I have noticed over this course with Shinji is that he really does pay his debts. Heck you can argue that everything he does for Sion is him paying the debt he feels he owes her. We do owe a debt to her and another one to Touko and this way we maybe can cover both in one stroke.
Or maybe Shinji just racks up debt after debt with her as he Elesa learns the joy of Shinji the way that early Ayaka saw him.
Yeah I been going back and forth on to have it in or not. I can see it being important or being something that raises an alarm in her mind when he really does not want to say why.
As for your ideas Daiki on support, I can get behind them, thought I am not sure Lestrange needs a knife. And giving her the recipe for Shinji's fading is kinda necessary all ready if Shinji going to get her to help improve it. Fading potion dose certainly could help her. Maybe giving her some notes about eastern herbs to use while he is gone with Elesa. The sunlight potion might help her but I am leering of handing her if we are going to go adventuring in some dark hole.
We could give her the ring of fireballs...oh wait never mind no rings to her. No one would ever let that go with Shinji giving her an ring.
Ok rewrite time for Lestrange's write in. Anything more people want to add/change in this? I am not sure we need to give a large amount of resources to her, she has her own. But if Shinji is going to go off with Elesa we need to at least hand/help her some stuff to at least give some aid before Shinji can really help her in the field.
[] Full disclosure of Shinji abilities. Lay all his cards on the table including his fading potion, so that she has a better idea how he might help or request his help in the future. Inform her that he owes the American champion a favor and that he and she will explore some of the ruins of the island, so he might be occupied doing that. But afterwards, he plans on helping her full time. Give her a dozen Flow Ofuda. Give her some notes he has on useful eastern plants for her to find. Give her a dose of his Fading Potion.