A beef I have when talking with other leaders is in negotiating for tradeable items.
If Victoria wants -for instance- dye off me and I'm unwilling to trade it, Victoria gets to express her dissapointment with me refusing, which can be strong enough to make me rethink my answer. At the very least I oftan give them a small amount of gold to keep them happy when I've turned them down.
But if I want something off Victoria that she's unwilling to trade, I can't show my dissapointment to her because I can't evan get the item onto the trade table - like she can with anything I have.
I think this gives the AI civs an unfair advantage. I'm always dissapointing them, and when it's visa versa they are never much aware of my dissapointment in them not wanting to trade. The human players should beable to guilt the AI like the AI can us. The AI's should know not take my support for granted.

You don't get any negative modifiers for refusing a trade. You only get those if you refuse to help them "in a time of need".

yes, I think all diplomatic options can be available, at any time. Even though -for an example- Napoleon has only one pig resource, you can ask him for it. Even though he's just cautious and all. Then he'll just say no :D

If I want his pig I can go to the trade table. If it's colored white then I can put it on the table and they can accept or refuse from there. If it's colored red then he won't trade no matter what and I can't get it on the table.
It could be in a time of need for me and I still can't put that across to another civ, if the item is colored red(off limits). So they still have an unfair advantage.