Does your 850 even have enough pci-e connectors?
Gaming eats a lot more power than hashing.
Yes, but that is only because CPU power in gaming.
Not only. Cards also run less hot when hashing vs when gaming. You can test that by running low cpu usage bench. Amount of resources used differs a lot.
Anyway, my best advice to anyone would be - even if it runs stable for 10 hours, do not leave it unattended for long, or without some fail-safe solution.