Will marinating chicken for 18 hours make the meat fall apart?
It'll be fine.  I've done this multiple times, even occasionally for more than 1 night and never had a problem, if anything it improves things as the marinade has time to work into the meat.  

If you can, give it a stir a coupe of times (every 6 hours maybe?) to ensure even coating.  

The acids in orange juice and most (?) marinades are generally too weak to break down chicken into component parts.