It's not slower than SRAM for certain amounts up.
Not sure what you mean, if you're saying that the capacity makes up for the lower speed that's essentially what I was getting at, I'm just not sure how it would scale for a gaming use rather than a server use. The reason I say it's slower is that eDRAM has to refresh its memory banks, eSRAM doesn't, so for timing reasons eSRAM ends up faster.
I don't know either, but it was certainly more R&D effort to go with eDRAM and the chip would cost the same if they used 1MB SRAM, so 3MB eDRAM being superior is the most logical conclusion.
Not necessarily, as mentioned IBM already has chips that use eDRAM so they don't need to reinvent the wheel there. It's possible Nintendo just wanted 3MB cache period, and then for cost reasons chose eDRAM, rather than performance reasons.