The official answer is approach the Apple Store fellas and have them help you on this. This article will answer some of your questions
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The answer is most likely NO, unless the PCI-E board of the SSD storage is removable. Wouldn't know until someone start dissembling the latest MBP series. You are encouraged to get the SSD size as you anticipated to be adequate for your daily needs. If you need more, offload to an external USB3.0 drive or a NAS
8MB or 16MB will not serve your needs. 8GB or 16GB will be more reasonable

Basically 8GB can carry you far. 16GB is a tad too much for most consumer, however, if you can afford it, why not. I'm using a 17" 2009 MBP series and is on 8GB and have been doing numerous photo editing tasks on both CS5 and Lightroom 5. Memory is often not the reason why you get slow down at 4GB and above. It's often your disk and your computing power if you feel slow in some filter stuffs.
For video editing, memory will be your biggest issue, computing power is. If you can afford a Mac Pro, go ahead. If not, live with lower expectation and don't edit a 4K video.
Certainly for the kind of tasks you have in place, Retina display will give you more screen assets for editing. Between 2.8 and 2.9Ghz, doesn't make much of a difference. Turbo boost will most likely not kick in if you are already using multicores intensively by your application.
Quad-core will be faster if your application uses all the cores, which you will most likely be using for video and photo editing.