1. As a main character, it is looking unlikely. In the Masamitsu Hidaka interview, he stated they prefered to switch the female of the group with the one from the newer generation as they are more easier to customize. However it really isn't a firm no. But for now, let's just hope for a Misty appearance in Sinnoh.
2. We don't know, Delia just found Brock one day and when she asked what happened between him and Professor Ivy he went into his sulky "I don't want to talk about it" state.
3. Yes, he did appear in Johto for 3 episodes. These episodes were EP187 "Moving Pictures", EP188 "Spring Fever", and EP189 "Freeze Frame". He was there to take a picture of Articuno, and once he did he decided to stay there a bit to take more pictures of the Pokemon living arond there.
4. It's a popular rumer, however the Pokemon Company nor Nintendo has ever confirmed a Generation II remake. There seems to be hints that they are or were at least thinking about making a Generation II remake, but at this time nothing official has come out about it. If you want ot read what these "Hints" are, you can got to Bulbapedia's article on it HERE. Incase you're wondering, fans are calling is GSDS, for "Gold & Silver Nintendo DS".