It's usually obvious when you've got a werebeast on your hands, but having some feedback on the ground would be nice too. Unless the werecurse causes them to black out and forget what happened, I agree that dwarves should attempt to isolate themselves when they suspect their time of the month is at hand. It may prove ineffective when the werebeast starts busting down doors and hunting people down, but acting repsonsibly counts for something. However, straight up telling everyone you're a monster sounds like a good way to get lynched, and is perhaps left to overwhelmingly honest or lawful dwarves to decide to come forward and face the consequences.
Vampires can already feed without killing the victim, it's just a rare occurence in fortress mode. It seems like vampires wait until they need to drink enough blood to kill someone before they go looking to get their succ on, but there could be more going on. Reducing the amount of blood they need from each feeding, and possibly increasing the frequency of their feeding sessions might make them more manageable. Another solution would be to let vampires target pets and livestock, although this would require either making animals actually sleep, or allowing vampires to feed on conscious victims.
Necromancers and undead in general look like they're about receive some major upgrades in the next release. One thing mentioned was intelligent undead, which may just be what your looking for. Of course, whether 'intelligent undead' means 'intelligent enough to not attack non-hostile mortals' or 'intelligent enough to speak while attacking every mortal creature in sight' remains to be seen. There's also the problem of what kind of undead fotress necromancers would be raising and whether or not players can influence that.
Personally, I feel slabs and books should give an option to gain the power of necromancy rather than forcing it on the reader as soon as you open the book. In fortress mode, this reduce the chances of an unexpected zombie apocalypse because your bookworms just don't feel like learning necromancy, and in adventure mode, this would ease my rampart paranoia about opening a book and suddenly having powers I didn't really want at the time.