Input protected? I don’t know about that. But it should handle any reasonable output from another device (an exception being modular signals, for example, well above normal audio levels)
Yes, it just shorts the right input to ground
Yes, you’ll just get nothing on the right channel of the stereo cable.
I wouldn’t assume so. Some things are okay with it, others not. I wouldn’t do it unless it specifically says it’s okay.
It amazes me to see guys with thousands worth of modular using cheap crappy cables and adaptors, or those stacking jacks, of course it will often work ok but then the one time when something goes wrong the expense of the repair will be more that using the proper cables in the first place, a false economy.