Once you have inserted an MS+ card and adjust your speed, that speed is set on your machine (you do have the option to change it still).
Yes, you can emboss with all cards. You just will not have a 'shadow' setting to cut out the embossed image. You'll have to hand cut it with the non-MS+ cards. I think the ink tips will work much like the embossing tips.
As far as the magnetic mat goes, there is a cover that goes over your Slice that magentizes it to your mat. Wherever you set your Slice, it will be stuck to the mat because the entire mat is magnetized. It is an entirely new mat with the embossing mat built into the reverse side.