Facing the prospect of never having time alone in our own home for 6 months?

I think of this is based on fear but I doubt very much your FIL intends to be with u every second of the day. i am sure he understands that u will need time together and will most likely try to make himseld scarce as to not get in the way. The best thing to do is to be open and honest from the beginning by saying something like. " Dad sometimes hubby and myself may still do things together alone, i hope u understand that this is a need we have and that you wont feel left out or unloved" i bet he says "of course sweetie, i dont want to intrude anyway"