Hi GQAdonis,
You ask a lot of detailed questions, I'd love to be able to answer them all in detail but I'm drinking from a fire hose already so briefly I will answer what I can.
It is very easy to add custom features and integrate your own tables in mojoPortal. I think with any CMS you are going to have to invest some time to understand the system in order to become really comfortable developing custom features or solutions. One of the great things about open source is that you can see the source code for existing features and find examples of almost any specific thing you need to do.
We are always working on improving the documentation. In addition to the developer docs section of the site we also have an online API reference and you can also download a .chm file with the API documentation from our download page.
We do have a rest service for authentication, but it is the one built into ASP.NET and based on the MembershipProvider, so it does not currently support oath
http://demo.mojoportal.com/Services/AuthenticationService.svc
We also support the RPX service which uses openid/oauth, I just tested and was able to login using my open id from my iphone on this site. We also degrade the editor to a text area automatically for the iphone since it does not support wysiwyg editors, so you can even answer forum posts or edit content from the iphone, though it requires knowledge of html to do extensive content editing since there is no wysiwyg.
Best,
Joe