Well, I hope you all don't hate me for saying this but, I believe EB's letter has truth to it.
It just fits now.
My theory:
AB & EB find Z dead in her bed (from neglect, abuse, natural causes, whatever). They PANIC! AB disposes of the body in a terrible way (wood chipper, cut it up, burn it, something along those lines) while EB gets the blankets, pillows, etc. They go out, buy a new mattress, dispose of the old one. In the process, they throw out the leg.
When they return home the night of the fire, they start their plan. Putting the pillows, blankets, etc in the car, with hopes of burning the evidence. The fire fails, they have to call 911. This is when the ransom note comes into play. I do not believe they had intentions of ever calling 911 or making it seems as if it were a kidnapping but the accidental fire made them panic even more.
EB's call to 911 does seem like she's in a panic, while AB's call seems like he is ordering a pizza. I believe that HE is the one who came up with the plan and she went along for the ride (she IS just as guilty though). There is no way any loving, caring father could make a 911 call about their missing child and show not one bit of emotion other than LAUGHTER. Even if the call was a fake story (which it was), he should have sounded nervous at the very least.
Just my opinion of course but after the letters, it fits to me. It really is like putting a puzzle together and we keep getting the pieces slowly and now, it is fitting to me.
raying: 
