Aye; but the Protheans were the last civilization to build on it. And I do not know the others to name them. Rule's lawyers....
All that said, it was the Catalyst that seemingly created the switches, even the ones that supposedly were opposed to their own interests. This speaks of objectivity to me. But if one suspects deception, why then pull the switch that supposedly destroys them? Rhetorical; they will come back anyway....
Those are excellent questions. IMO, that's something that can only be explained by "plot reasons".
As players, we can only take an educated guess as to why the Catalyst (or the Intelligence) would show the Reapers' off-switch to an organic, stuck in the cycle.
My guess would be based on the Intelligence's logic that it couldn't find a REAL solution to the organics-synthetics mutual destruction problem. It knows it found an imperfect solution, for it's obliged to destroy life in order to preserve it. The ideal solution would be to find a way for organics and synthetics to coexist, a solution it failed to provide.
So, from the moment Shepard is able to kill two Reapers, succesfully unite the galaxy against the Reapers and arrive at the Catalyst's chamber, three fantastic feats no life form had ever managed to achieve, the Intelligence saw that their solution was no longer working. Organics managed to create minimal conditions to turn the tide to their favor. How long would it be until the civilizations of the next cycles would finally become advanced enough to destroy even the Reapers? To surpass their technology and finally bring them down?
This is why the Intelligence surrendered. It realized its provisional solution could no longer keep up with the galaxy's natural evolution. Organics would eventually become aware of the existence of cycles, understand Reaper technology and destroy the Reapers for good. So it relinquished control of the galaxy's evolutionary cycle and allowed the leader of the fight, Shepard, to hold the fate of the galaxy in their hands instead.