So...as I have been exploring the new domain mapping further, there are HUGE BUGS that need to be addressed ASAP as the plugin breaks site functionalities. The default set up of "directed to mapped (primary) domain" isn't working, with a multi-site network with https enabled, here's why:
This will cause a blank screen. This is how most people will attempt to access the customize feature because they wont be able to visit it via http://networkname.com/subblogname/ because that will forward to their domain
The proper functionality (as you had it originally) was to not have https://networkname.com/subblogname/ forward to https://subblognamedomain.com/ and instead just have it ignore the forward and instead have it only forward when visiting http://networkname.com/subblogname/ like it did before.
A work around is to have customers choose "disabled and entered domain should be used" however there is not mention of that feature after the domain has been added making it very difficult for customers to figure out to delete the domain for the non-forwarding.
Anyways...it would be very helpful if these things were addressed soon as they are causing a lot of confusion from a customer sub-blog prospective.