Build out of Mobile Joomla..

Can you please tell me if I can build out Mobile Joomla.. just allowing admins to see it? so that the public does not see the startup and corrections we need to make?

Thanks

You can disable all "mobile" plugins (except of system "Mobile Joomla" plugin) and switch to mobile version using "yoursite.com/?device=xhtml" for Smartphone mode and "yoursite.com/?device=xhtml" for iPhone mode.

So your basically saying that I can be working on it on the live site and not allow anyone else other then admins see it by placing the url variables behind the domain name?

Yes, mobile version will be displayed for users who know "device=" parameter.
Note: disable "Select markup" module too (it is designed to switch between mobile version using "device=" parameter).

If you like to enable MJ for admins only, you can change "access" level for MobileJoomla plugin to "Special".

I understand it is only designed for bas-line Joomla 2.x-3.x.. we have some 3rd party extensions like Sobipro and Kunea forum... how do we go about testing with spending the $100+ ahead of time to find out it won't work?

You can test with Community version of MobileJoomla. MJPro doesn't change the way your pages are displayed (with respect to Community version), it adds support of retina displays and tablets..

So after I install the community version, and if it works properly.. how do I go ahead and upgrade to the full version?

MJ Pro can be installed over MJ Community. All settings will be kept.