domain mapping on cpanel strange behaviour

Hi, I just signed up yesterday and am trying out the plugins and configs, and I'm trying to get the domain mapping to work on my server. Let me describe the scenarios and see if you can help me figure out what is wrong!

CASE 1:
I set the DNS for the domain I want to map in the registrar to point to the MU cpanel, installation account, then add the domain as a parked/addon domain = it works

CASE 2: I set the A record at the domain registrar to the MU cpanel installation IP = it doesnt work, no mapping

CASE 3: I set a CNAME or A record from ANOTHER cpanel to my MU cpanel IP = it doesnt work, no mapping

I'm not sure if I'm doing something wrong, but I need it to map from other registrars or cpanels, without me having to add the NS or adding the domains as parked/addon in the MU cpanel. Is this even possible?

Thanks!
Sam