would summoning a lemure qualify you for tainted sorcerer?

[Lo'Tek]

It's semantic lawyering worthy of a deal with a devil to argue that summoning with the intend to gain instruction would be enough to fulfill the clause as written, even if the summoned creature is unable to provide the instruction.

However i find the rules intention quite obvious: the context establishes clearly that a teacher is needed, like a sorcerer who already practices blood magic taint sorcery. The alternative of summoning a devil to get forbidden magic / knowledge / power is a classic and stands as an example. The act of summoning is just a means to an end, the acquisition of knowledge from a knowledgeable source the actual point.

More so the primary purpose of the rule is to prevent it from being picked up arbitrarily on a level up without fluffing it up.

Interpreting that rule as such I would rather expand the list of possible teachers to include yugoloth as well as certain books written in blood, then allow a mindless lemure to formally fulfill it. You might be able to "summon a lemure" but you can not "summon a lemure to gain instruction".

(do) i need to grab fiendish summoning specialist and add a mane to sm ii's list?

No, a mane isn't much better, it is the same in red. Like the lemure they are tortured souls, closer to petitioners then to actual fiends and only qualify as devil/demon due to being native to the lower planes. They barely understand simple concepts and can't speak. These are not the kind of creature anyone would summon to unlock the hidden secrets of the multiverse. The lowest fiends are mere tools you would summon to slow down an enemy or to experiment on.

Something could be constructed based on the later: a mane/lemure might crave blood and by studying that interaction one could reveal how blood is linked to life essence and how life essence can be exchanged with the planes of hell for power, thereby unlocking the blood magic taint sorcery skill of substituting blood for material components. Ironically such research is not typical for low level chars and could be argued to also require additional skills, so other alternatives may be more agreeable with your DM.

IMHO, if you go for fiendish summoning for the purpose of arcane power, then go all the way: summon an imp, make it your familiar, and if you want real power, use it to get your hands on a ritual that grants an audience with an Amnizu (hells bureaucrats), a Falxugon (temptation of mortals) or Paeliryon (information brokers) or the equivalent tanar-ri or yugoloth depending on your alignment of choice.

i dont want to ... require dm negotiation.

then i would highly recommend pursuing a path that "just works" both in story and game mechanics, instead of one that seems forced and must be defended on semantics. Power flows through the path of least resistance.

i dont want to use fiendish summoning specialist

If that is the case, the whole "summoning a demon/devil" thing might be the wrong approach altogether and an "evil cult" backstory the far better choice for you.