Is installing a HDD instead of SuperDrive safe?
Overheating shouldn't be an issue. The SuperDrive probably runs
hotter (when it's in use) since it has moving parts
An SSD would most likely improve performance (especially if you use it as your boot drive and place for applications)
Don't know about battery life, but I imagine if you you use this as your boot drive, and the stock hard disk as your "slave", you might actually see an improvement (again, fewer 'moving parts')
This will void your warranty, but if you're careful you might be able to get in and out without leaving evidence of tampering. Consult iFixit's guides for tips
I can't imagine there would be a noticeable weight difference. 
Steve is dead, unless you're talking about Steve Zahn. Steve Zahn doesn't care what you do with your computer.