We don't exactly know where it will be tunnelled, Metrolinx has been mum on this. In one interview, Phil Verster did allude that it will be tunnelled past the Humber river, so I am going to go out on a limb and say the section immediately after Mount Dennis to past the Humber River will be elevated, but we don't know at this point.
As for the Pearson connection, my understanding is that Mount Dennis to Renforth will be built first, with the Pearson segment completed when the GTAA builds the transit hub/Union Station West. Without that project complete, the LRT has nowhere to go at YYZ.