The Scenario

I have a domain attached to my hosting plan and whose WordPress files are in ".../public_html/". Let say this is maindomain.com. I have bought additional domains (say addondomain1.com to addondomain18.com) which also run WordPress as addon sites (Let's say these are addon1, addon2, addon3, addon4, addon5 ... and addon18). However, I did not install their files in the ".../public_html/" directory like ".../public_html/addon1". Instead, they are in the same hierarchy with ".../public_html/" i.e. ".../addon1" or ".../addon9". I currently have 19 unique domains - 1 main account, 18 addons - all sharing the same IP.

The Problem

Recently, I decided to reorganise and streamline my websites. I grouped the domains into 3 classes viz: Personal, Family Business and Other clients. I installed multisite (sub-directory option, still in the ".../addon9" hierararchy) on 3 addonsdomains; say addons 3, 7 and 11. I created subsites for the others (including the main site) under these 3 multisites according to their classes. The objective is to be able to access each multisite from their unique domains with the address showing something like addondomain18.com instead of maindomain.com/addon1 to site visitors.

I have followed all the tutorials on Domain Mapping I could lay my hands/eyes on for two weeks now. Accessing the addondomains from their unique addresses e.g. addondomain11.com, always redirect to the main hosting domain, maindomain.com (if the redirect ever works).

What am I doing wrong? Is this due to my file structure: ".../addon9" instead of ".../public_html/addon9"? Kindly advise.