There nothing wrong with a geo/poison staff. Just keep a second staff on hand for when you fight undead.
Here is something you may wish to try, although it is more a ranged build than melee, it can still work in melee. I still recommend a couple points in Necromancy, then a point in Hydrosophist and then you can go nuts in Geomancy. With a point in Hydro and Necro you can get the skill Raining Blood. You can cast Raining Blood where you intend to start a fight, then cast Poison Dart into the blood. Now stand with one foot in the blood and one foot in the poison. As an undead, the one foot in the poison will keep you healing. In you have the Elemental Affinity talent, all of your Geo and Necro skills will cost one less action point to cast. This can allow you to go nuts with skills the first couple rounds of combat. If you don't want to do the Necro side, this all still works just doing Geomancy. You can use oil surface to get Elemental Affinity with Geomancy, but that will slow your movement and doesn't heal your undead like poison will. However, if you are going for Medusa Head you will need two points in Polymorph. This means you could use oil for Elemental Affinity and then take Spread you Wings to move around combat. Another useful Polymorph skill to move around in combat is Bull's Horns. With two points into Polymorph, you can also get Heart of Steel to regenerate your physical armor.
Even if you don't want you use Necro skills with the build, I still recommend putting two points in to Necromancy. First, as you've already mentioned, this gives you a bit of self heal from attacks. Second if you combine a Geomancy skill book and a Necromancy skill book you get the skill Corrosive Touch. This is a one AP skill that will set acid on a target for two turns. During these two turns, the target will lose physical armor based your characters level and intelligence. Unfortunately, it does come with a hefty 5 turn cool down. If one of the two skill books is a source skill it will create a book for Corrosive Spray instead, which is a cone AoE instead of a touch skill. However, this requires two points in both Necromacy and Geomancy to use. When used it consumes a point of source and two AP. This seems like something that will fit what you want to do with Fane. Finally, I would still recommend picking up Mosiquito Swarm. Healing an undead can be tricky and the self heal for Mosquito Swarm is great for a character with high intelligence. Try going without it first. If Fane is getting killed then, pick it up to help keep him alive.
Keep in mind, part of the enjoyment of replaying the game is to have fun with different builds. Keep working with this one as you can make almost anything work. I had to play around with things quite a bit to get a witch build I thought was true to the preset. At first the build didn't play quite the way I wanted, however, as I got to higher level and had more points to develop the character, by level 13 or 14 it was doing what I wanted it to. The point is to keep a goal in mind with your build. It may take some levels to get it to play exactly the way you want it to, but when it does, it will all be worth it.
