ACE loads the plane in that it rewrites the config file.  They do want you to check the fuel/payload page in FSX.  That was their design decision for whatever reason.  It is different than some other products, but whether you are looking at a page outside the sim or one inside the sim, there is no practical difference to me.  Of course, everyone is free to have their own preferences.  Sorry this spoils it for you.

I'm used to it now, so it doesn't bother me.  Actually, I like knowing how FSX is "seeing" things for certain and have started using the nav log as a fuel planner for other aircraft, as well (adding appropriate minimum fuel and covering contingencies).