Kerry, the microwave and hot water serve the same purpose as scraping the sides, which is to heat the immediate exterior of the container and thus loosen any ice that would've adhered to the sides, allowing it to get spun and broken up.
Randomwalk, results seem to vary machine to machine based on my reading on reddit and facebook groups (and this thread). There's also differences from user to user on freezer temperature. This thread seems to indicate that running your freezer at the lowest temperature is desirable. Mine is currently at 0F/-17C which is the middle of the range recommended by Ninja Shark (a broad range of -7F to +9F).
I think some of the ice crystals might be user error and they're not bad enough to make the result unpalatable, but like most on this forum, for me, less than perfect food is well, not perfect.
Custard base shouldn't require any stabilizers to my understanding since the egg serves a similar purpose with lecithin and fats.
