LB soul training? hell no, even if she isn't lying and that's how it works you are naive if you think that's only what she is doing and there isn't any catch.
The NS guy can attack your soul and TW remnants messes him up. Aren't you curious why it didn't react the same way against LB?
There can be several logical interpretations of why. As like because LB is a Immortal with a proper Core-Formation level Avatar and, therefore, can use the powers from the Soul better than a Nascent-Soul in a I-Star body.
Or even the origin of the attack, as Shengjue attacks our Soul with the full intend of destroying it(As he does if you don't have Iron Will/Formed devil).
And, as LB said, about the manuals, Wu Heir can be able to improve them futurely, and they still are Immortal Magic. The closest thing is an Yue aligned Wu Heir rejoicing in a Void-Grade manual. Meanwhile the LB aligned Wu Heir already has access to three Immortal Magic(RAF, and the manuals). — Also, clarifying, Immortal Magic is the highest Grade thing we'll get our hands over in PoMA, as Divine-Miracle is more of a "Trademark/ultimate Art" of an God, by Niki, and each God has only one of it. So naturally we hadn't saw TW's Divine Miracle yet. So perhaps we can get our hands on his Divine Miracle somehow, who knows.
Anyway, there's no such thing of slave route, we can be at first, but LB's power are still limited by the world, as she said, she only has one thousandth of her memories and still limited to Core-Formation.
Once Wu Heir reaches Core-Formation you really think we wouldn't have leverage? Or even before, with TW's defences on our Soul(Be it pieces of his self represented by the voice who helped Wu Heir before or something else). Be appealing for logical countermeasures or appealing to the extraordinary(TW, Enlightenment, something from our Dao Seed...etc), Wu Heir is no one slave.