In the implementation of the School Understanding ability of the arcanist in terms of the abilities chosen, you gain the ability of Diviner's Fortune and are able to use it 3 + CHA Mod times per day as if you were a 1st level wizard. Which means only +1 to the insight bonus.
By expending 1 point of the arcane resevoir as a swift action, you increase your "virtual" wizard level to be equal to your arcanist level insofar as Diviner's Fortune is concerned. At the same time, for a number of rounds equal to your charisma modifier, you gain the forewarned ability aka "the other ability gained at 1st level for the chosen school".
That is correct. The OP was talking about taking the Forwarned as the implementation of the School Understanding and that is what I was referring to. You cannot take that feature because it is not a 3 + Int Mod type feature therefore it does not fall under the ability. However, as you stated, once you spend the arcane point, it is active for a number of rounds equal to your CHA mod.