A few general tips, which you probably don't need but here they are:

1) Just like in D&D, it's better if you have very few stats you focus on and a lot of stats you "dump", or ignore entirely. Going for Scoundrel / Geo, for example, means you'll want at least a little of Str, Finesse, and Int, as well as of course Con being great, Wits to help initiative since you're focusing on sneak attacks, and memory for all the skills you'll be using.

If conversely you were say Necro / Geo, you'd want Int and Con, and maybe Memory as needed, and could ignore the rest. 1/2 the stats to focus on means 2x the amount of those stats and thus stronger spells which (thanks to Necro) heal you for more.

2) The way you easily and effectively "win" is through winning the action pool battle. More AP, or more effective use of those AP, wins every time. Thus things that grant additional AP or turns are top priority, as are things that actually let you Do Something that matters during those turns. Having 8 AP is great, but if that's just 4 auto attacks for 5 damage each it doesn't help that much. Just something to keep in mind.

3) As undead, you can heal through poison, sure. You can also heal through dealing damage to the enemy's health (vitality) if you have necromancy magic (you get the healing regardless of the source of damage you use, it doesn't just apply to necro magic). You can also heal from necro spells like mosquito swarm, talents like leech, poison potions, or using bedrolls when not in combat. Don't think poison is the only way to heal up!

4) my personal views on each tree. Mind you so far I've played around rather extensively in Dungeon Master mode and have started two playthroughs, one just for fun with a friend and one where I'm testing out my theories as to optimal vs suboptimal builds. It will be weeks before I have final conclusions regarding what works well and doesn't in actual gameplay, as it will be for anyone else who is doing methodical and scientific experimentation, so take everything said with a pound of salt. Here goes:

All weapon abilities:
These provide a damage boost with a particular weapon set and a minor ancillary bonus. Though a lot of people put a LOT of focus on "moar damage" (and indeed the best debuff you can apply to an enemy is "death"), they provide the same bonus as boosting a stat, or a skill, which provides a damage increase, and often has benefits of its own besides. Very low priority imho, I'd never go for this unless you are absolutely 100% damage focused and even then I'd probably not.

Leadership:
Often associated with the summoning tree, this can be Very impactful. However unlike most trees it becomes More beneficial with each point invested rather than less. Going lone wolf and getting this to 20, for example, gives all allies a 40% dodge chance and 60% to all resists, cutting damage in half. However for obvious reasons the number of allies who would actually benefit at that point is minimal, as is the benefit of very few points invested in here. Also keep in mind that the person with the investment in leadership does not themselves get the defensive benefits. Still potentially useful, just only if you're doing so on a character who will max it out asap and there is no more impactful choice for them.

Perserverence:
Relies on 1) your armor being wiped out, 2) being hit by a particular type of cc, and 3) recovering from that CC to do Anything. Ideally you'd avoid 1 or 2, and there are other ways (such as skills) to recover armor which don't chew up vital ability points, which are far less conditional besides. Maybe others have better experiences with it but I'd skip it entirely.

Retribution:
For the same reasons Thorns builds in Diablo 2 don't often work I'd not recommend this either. Enemies will very typically have far larger health pools than you, so even if you max this out, dealing 50% of the damage they deal to you back to them is not an even trade. Increasing the damage you yourself deal so you can kill them and they don't deal damage at all is a far better idea.

And finally, the skill trees:
Aero, Geo, Hydro, Pyro:
All elemental damage can be resisted by monsters with the appropriate resists. Just keep that in mind. All elemental damage also scales with Int for damage. All trees also provide various utility skills (Geo the least of them since it has both earth and poison in a single tree). Aero has movement based abilities, Pyro has self buffs, and of course Hydro is full of heals and restoration effects. Keep in mind the heals from Hydro scale with points invested but not with Int, so Hydro is often a good tree to pair with someone who is not Int focused as long as you're not focusing on the damage effects from that tree. Getting more points in any given tree increases the damage done with spells from it, typically, with perhaps a minor effect on top of that.

Scoundrel, Warfare, Poly, Huntsman:
All of these trees focus on physical damage done. Scoundrel is unique in that 1) it restricts you to only daggers, mostly, and 2) it has skills that key off of both strength and finesse. Just keep that in mind. Warfare actually increases physical damage dealt, and thus is useful for builds that deal physical damage, regardless of what type (necro, huntsman, or any melee character) of physical damage dealer they are. Polymorph also essentially increases damage per point as it gives attribute points, which can in turn be dumped into Str, Finesse, or Int as appropriate to increase damage. Huntsman's effects are conditional, requiring you to be on higher ground, and thus are very map dependent. Warfare and Poly's skills are almost exclusively keyed off of Strength.

Necro, Summons
Setting these aside because they're both rather different. So Necromancy is physical magic damage, and thus can't be resisted, and gets bonuses from warfare rather than investing in the tree itself. Investing in the tree instead gives you life leech effects for ANY damage to the enemy's health you deal. It also has summons in it and a large number of utility effects including death immunity. Not to be underestimated, but unless you're building right the damage can be very subpar as it relies on investing in a tree other than this one to power up, which you are unlikely to use very effectively in terms of the actual abilities themselves (as they key off of weapon and str while necro damaging abilities key off of level and int). It actually pairs very well with undead, but between the two you're entering a World of complexity, so just make sure that's what you actually want before making that choice.

Summons key off of no attributes. Instead they gain benefits from simply investing in the skill itself, though of course the summons benefit from leadership or other AOE buffs. Note that you can only have one "major" summon at a time, so even if you cross between Necro and Summons and can summon a bloated corpse, skeleton, condor, and Incarnate, summoning any of those will erase your existing summons from that list from existence. As such you can only really summon 1 thing, plus 1 elemental totem per turn (max 3). Just be aware of that.

4) When it comes to complementing each other, remembering all of the above, keep in mind that damaging someone's physical armor to nothing, then having other characters focus on his magic armor, means you're essentially giving him free hit points. Having characters which deal all physical damage and have a bunch of CC to use which keys off of physical armor is a great composition. Having characters who all deal magic damage (of a variety of types to get around resistances) and have a bunch of CC which keys off of magic resistance is a great composition. Mixing and matching, though still possible and effective, may be a bit less effective.

All that being said, for Fane (or any undead) I really like focusing on Warfare and a split of Polymorph and Necromancy (focusing on utility spells and summons not damage spells) if you want a melee character. You get to deal a bunch of damage, have CC which keys off of phyiscal armor, life leech back health whenever dealing damage to keep yourself topped off without wasting actions on it, and it's very thematically appropriate.

If you are going a spellcaster route, Geomancy and Summoner is fine. Just make sure you have a pyromancer in the party to blow up all the poison / oil fields you toss onto the enemy, and that the rest of your team is focused heavily on magic damage and crowd control keying off of magic damage. Be aware that due to both resistances and less "hard" cc, as well as lower single target damage and more AOE effects (if you kill someone you eliminate all their actions, damage does not, and eliminating enemy actions or increasing your own is how you win), a magic team may struggle a bit compared to an all physical team. It sure is a hell of a lot of fun though and as long as you're not playing on tactician+ it really shouldn't matter.

I know that was way too long to read, good luck and have fun.