1. Yes, hexes are useful. But usually spells are more powerful. So you could start with one of those and throw your evil eye once the combat is in full swing. Also: If the combat is over quickly your hexes probably didn't make much of a difference, but when combat drags on they get more and more useful (because you just have to keep cackling to keep them going). Fought a rogue with misfortune for 6 or 7 rounds - poor bastard couldn't hit us at all.
2. Be very aware that there are scenarios with only or mostly undead. So any spells with Fort as a saving throw and mind-affecting spells are right out. So almost everything you bring to the table - be prepared for those occasions. (One scenario had humanoids which were immune to mind-affecting spells, vermin and undead - our witch had a pretty bad time, unfortunately).
3. You do have non-combat spells as a witch. If you decide that hexes are what you want to do in combat primarily,
4. Personal opinion: I hate slumber. Either it fails and the witch is grumpy or it's successfull and many combats are over in a heartbeat, considering you often fight only one or two enemies.