ghosts only appear if they have died in horrible or sudden ways or if they have unfinished goals in life meaning they need closure with themselves to move on there is also the fact that some angels may look like ghosts to blend in and some are poltergeists.
yes however if they were able to come to terms with their deaths and trust that Harry would live then they would be able to move on, basically if they saw that Lily´s sacrifice worked they would move on