To answer some of this, we'll have to wait until someone does a detailed upgrade video or blog post with pictures. If you were just swapping out drives, I think it'd be really straight forward. I'm not sure if the second slot is already cabled (probably not), or how hard it is to get to that slot.
Have you considered buying a USB3 or TB external enclosure, dropping an SSD in there, and then using that as your boot drive? I know it sounds like a hassle, but a friend of mine is doing that with his 2012 iMac, and the machine is incredibly fast. As fast as any PC with an SSD I've ever used. Throughput on USB3 is so fast, I doubt you'd be able to tell you were running off an external. That way, you could at least keep it in tact until your warranty runs out (assuming they are giving you a new warranty on a replacement computer).