1 covered above.
2) The psychic will give you a hint on the office girl - you need to find her in the hills on a monday, and find out some interesting stuff about her.
3) You can buy a diamond outright from the dwarf in the mines - ask to sell gems and click the buy option. 4000 toads, though. I believe this option was added because EVERYONE got fed up of trying to dig one up.
4) There are six that I know off that you can hire. The twins, mermaid and dryad you cannot, unless I've missed something.
Alma, Issa, Mazzy, the woodcutter's wife, the girl who wants a diamond, and the whore/beggar in the street near the equipment shop. (Hire her services a few times, then you'll get the option to hire her permanently.)