NPCs are not absolutely stupid or oblivious. They live (usually) in a world filled with magic. There is no reason (usually) for them not to be aware of the possibility for such, and therefore they should act accordingly.Two things. First, Xanathar’s is all optional content. It’s not RAW in the same sense as the PHB. Second, guidance is an action to cast with concentration of 1 minute. Cast it before you enter then negotiate. Negotiate for a minute or two, step outside, get another guidance, go back inside and negotiate some more. Overcoming the “roadblocks” you suggest are trivial.
I don't do +10, I give advantage/disadvantage because I like that mechanic better. Or I just make the NPC hostile "You get yourself right out of here! I don't know what your friend just did there to ya, but I know you are up to no good! Get before I call the guard on you, ya mangy daughter of a hyena!"This is the DM making a call. Nowhere in the rules does it say anything about a +10 to Insight or tossing the character.