/tradeoffer or /offertrade should bring up the correct switches for you to use. I am not at a point that I can check in game, but I used it the other day just to see how to do it. Hope that helps.
Yes, you need characters in each realm and the command can be used from any realm, no need to wait on the realm timer.
Just log in your mids if that's where you want to play and estimate how many plat you have in hib. If it says you don't have that much, drop the qty until it goes through.