How should I prepare subfloor for backerboard installation in bathroom tiling project?
1) Safe assumption.  Your floor trusses are carrying the load of the entire room.  A little extra tile weight isn't going to matter.  

2) You want a relatively level surface but the thinset itself will have enough flexibility to allow your final floor to be perfectly level, provided you take the time to make sure each tile is placed level.  If your variance is more than 1/8" you'll need to apply more/less thinset based on the relative height (lower subfloor - more thinset) in order to ensure you can get level.  If you want to remove all doubt, a thin layer of self-leveling concrete poured first (seal all holes!!!) will give you that level surface you want.

3) I personally wouldn't have put any gaps between the subflooring as long as the material is the same.  However the thinset will fill your gaps - and you don't want to fill the gaps around the outside wall because those do serve a purpose for expansion.

One other note - this is a bathroom floor right? PLEASE apply waterproofing to the floor before you proceed.  Water will penetrate grout (unless it's epoxy) and get trapped, and if you don't waterproof the wood then it'll rot.

Also - related to my comment above - you don't need to use backerboard.  Backerboard is for wall mounted tile, to be used instead of drywall (it doesn't rot).  However, as indicated in the comments below, using it is fine and may in fact be recommended as it's a better surface for water protection than wood when under a floor.