Yes that is what I bought it for it did a great job with my Makita 18 volt drill. I did use a stick to free clumps around edge of bucket.
I’d say the diameter is about 3” wide
I wouldn't, it's on the small side especially if your mixing the whole bag. If you're only mixing in small amounts it might be good
Only if you are very patient.
Yes. It's very sturdy.
Sure, if it all you already had but I wouldn't buy it for that purpose. They make others that would do a better and more efficient job