The answer to question 1 is exactly like you state. There is a higher FOV and the fact the engine has to render everything twice one for each display.
The answer to question 2 is that it can not be changed. The Vive Runtime controls the fps and it adapters to the headsets refresh rate to keep it consistant. It will attempt to cap at 90 but if at any point your games fps drops below 90 it will drop down to 45 fps. This is because the headsets runtime will fill in the other 45 fps missing with its own frames in an attempt to hold that 90 fps to prevent motion sickness. There is no way around that but as it is a headset safety measure to prevent people from getting sick. Anything below 90 fps in the headset will result in drifting when you move your head around and that will make your customers toss there chucks faster then anything else.
So in short its just the headset doing what it needs to do to maintain a smooth 90 fps for the headset. While your game may not show 90 fps (if render drops below that) it will still output 90 fps in the headset displays. The runtime just adds in a slight movement to the image to keep the display headmovement smooth.
I believe that is the article where they go into talking about it. Also the same thing will happen if the frametime gets to high. It will cut your fps in order to bring the frametime back down to maintaine that 90 fps in the headset.