Had an issue with mine, awaiting a new G751JT and hoping it is defect free.
To answer OP questions:
1) I haven't use an ASUS ROG before. I don't see anything on here that is Optimus, unless it is completely under the hood.
2) Steam button cannot be reprogrammed, afaik. You can reprogram the m1, m2, and m3 keys through the ROG software, that's it.
3) ROG button cannot be reprogrammed, afaik.