By beefing them up, I'm suggesting the grilles be thicker in dimension. You cannot divide the current grille because 'lights" in windows are never that ratio of height versus width. You're the one trying to turn a ranch into a farmhouse. A lot of professionals would not recommend that.

dropping the window down could well be odd on the inside. i'd leave it alone for now. get the outside work done. it may be a non event with so many new things to look at on the outside. how about just adding larger trim to the top of the door?

I actually do not like grilles but unless you are willing to change the garage windows I would just leave them all. Looks like a farmhouse to me. Not sure what you mean by modern craftsman but IMO the interior does not need to be the same as the exterior in style.

I would not ever place trapezoidal windows on a colonial home; they belong on a more modern or contemporary house. Quarter-rounds would be a much better choice. As far as casements go, they are definitely a pain if there is a patio outside. You could do a combination of windows to get the 72" you desire--a nonoperational transom above a tall double-hung would give you the light you want but keep to the colonial style of your home. As for muntins (grills) you could do 6 over 1 in double hung windows for a classic colonial-revival style. As Mark said, consistency in window style and placement is important in achieving a seamless look on an addition.