"Whenever an investigator casts a Spell, he may accept the Lurker at the Threshold's aid. If he does so, he reduces the Sanity cost of the Spell to 0 and automatically passes the Spell check (…)" I'm looking for clarity about the part in bold. It's obvious when you use it on spells that require a single success to function and further ones wouldn't improve it a bit. So what happens, when:

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  The spell requires more than one success to work, like Bind Monster.
  
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  Gaining a single success will make it utterly useless, Spectral Razor for example.
  

Logic would dictate that you gain enough successes in the first case to neutralise the monster. Even with minimal lore, it would be possible because of the +4 modifier. The second case is what bothers me. One could argue, that the Lurker will only make you pass just barely, thus it is a waste to bargain with devil like this. However, wouldn't such a powerful being have the power to fully charge those spells? Think about it, fist of Yog-Sothoth even has the_name within! Not to mention that those spells are inferior to the static ones anyway. At least the Lurker would give the investigators some real temptation!

So, what do you think? Has there been any official ruling on this one?

If the spell requires more than one success, then Rites will grant the caster the required successes to pass, since you're "passing" the check. Thus, Bind Monster on a 3-toughness monster would get 3 successes from the Rites ability.

Spectral Razor and Fist of Yog are successfully cast when one success is rolled, so the Lurker will only grant one success this way. It's thematically coherent, because the lurker is Yog, and you wouldn't expect Yog to cause the Fist of Yog Sothoth to become so extraordinarily powerful that an investigator could kill Yog with it.

Alright, that makes sense from a gameplay viewpoint. Thematically speaking though, I can think of at least two cases where Yog's agenda (as far an unfathomable, eldritch creature can be understood) goes against the rules:

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  Bind monster can, as far as I know (unless spawns are immune) instantly defeat the Dunwich Horror, son of Yog-Sothoth. Without breaking a sweat.
  
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  Yog may have enemies among the ancient ones. Not every old one can rule planet at the same time and the gods of chaos may decide to thwart each other.
  
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  +1 personal: I think those spell are way underpowered. A little taste of their true potential (which doesn't happen, even with a blessing) spices the game imo.