The theory maybe very well fitting, Idk. BUT:
Why did the family arrive earlier than originally planned at Kendall? ("Surprise" for grandma, who wasn't surprised at all, because she already got informed about it by a call.)
MFC get an unexpected early finish from work, I believe
Why did the FP have hastily to put their cats into care and pick up their children out of kindergarden ahead of time?
See above
Why did FFC take photos in the early morning of that "Bombshell"-Friday, when she could have taken them later in that weekend as well?
Why not? There were a lot of photos and video footage of William. It’s very likely they intended to document the rest of the weekend also.
Why didn't they take some photos at the MacD, if this weekend was for relaxed photoshooting of places, little William visited and liked?
It was a quick stop to eat before they got back in the road?
Why did the photoshooting concentrate on William, when both children are from the same bio parents and contributions to the mandatory storybook could have been of both children? (Just the child, who would go missing very short time after that, is the main object, it seems.)
We wouldn’t be privy to those photos as that would breach the child’s privacy.
Why did William have to wear his new striking spiderman suit from Bali? (Allegedly, because he made some trouble while dressing him for the day, what doesn't have to be true.)
This is actually one of the elements of the story I didn’t have trouble believing because that’s the kind of thing kids do. Similarly, I could imagine the strong feelings a child would have if someone tried to force them into a dress up outfit they didn’t want to wear. I had Iron Man accompany me shopping yesterday 
Why did the MFC have to attend an "important" online meeting appointment despite allegedly bad internet connection at Kendall? (They could have made the journey later instead of hastily and earlier? In terms of time,
the meeting wasn't worth mentioning at all, if MFC needed only an hour for driving to Lake Wood and back, the online meeting, getting medicine at a pharmacy and getting newspapers at a shop.)
I wondered whether the morning meeting was set up to accommodate the cancelled meeting the day before, the one that allowed them to leave earlier.
So, it seems to me, there was some preplanning to the whole "event" and preplanning makes no sense, if a fatal "balcony fall" happened out of the sudden.
The pre-planning was to head up earlier because they freed up sone time to do so. I don’t think it’s sinister at all.
IF MFC was involved in the whole story, I wonder about his great acting as a desperate father, who was crying looking for his son. Hats off! - I would like to say. What an acting performance
