I want to play IWD2 and create the optimal party for me.

Since the manuals and in-game descriptions in the older games had quite some errors, e.g.

BG1: wisdom doesn't affects saving throws
BG2: not only the swashbuckler descriptions was wrong
IWD: fighter specialisation bonusses and max number of attacks per round were wrong in the manual, I wouldn't have created a paladin if I'd known this before

I don't believe that IWD2 is error-free and I'm afraid that I make the wrong choices while creating my characters if I rely on the manual.

So my question is:

What mistakes are made in the manual or in-game description and which important facts are omitted ?

First of all, the Improved Initiative feat doesn't work.

There's rumoured to be a problem with the Spell Penetration feat, although I haven't noticed.

There are errors in number of spells per level, spell schools, opposing spell school for specialist mages...
Just ask your question and paste what it says in the manual. If there is a difference between what the manual says and what is found in game, someone here will rectify it for you.

Thanks for the offer, but I'd like to know if a spell works like described for each spell.
I don't want to copy all the spell descriptions from the manual.
And there could be thousands of important game play mechanics omitted in the manual.

Is there some place in the web where the manual or in-game description errors are listed ?

Here some specific questions:

1.Do you get 1.5 times the strength bonus when using 2-handed weapons or 1-handed weapons with your left hand empty ?

2.The spell animate dead is marked [evil] in the description. What does this mean ?
Can a good or neutral priest cast it ?
Can a good or neutral sorc/wiz cast it ?
Can a good priest of tempus cast it ?

3.How does armor affect druidic or bardic spellcasting ?

4.Which bard songs are combat only ?

I don't think the manual mentions Cleric as the favored class of Female Drow.

To answer your question #3, Druid spells are unaffected by armor, but Bard spells are Arcane and subject to spell failure chances.

Two major spell anomalies in the manual are the Ray of Enfeeblement (Necromancy) which is listed as L2 instead of an L1 spell and Circle of Death (Necromancy) which is a L6 arcane spell that is just not mentioned.
Most spells work as they are supposed to. Others might know better.

1] You only get the 1.5Str bonus if using a two-handed weapon. Using a single handed weapon with no sheild gives you no benefit.
If you are an arcane caster, you might want to drop the sheild in order to stave off spell failure. In that case you might want the Deflect Arrows feat which only works if you carry a single handed weapon with no sheild.

2] Animate Dead can be cast by anyone afaik. Being an arcane spell lifts that condition I guess. Same is the case with Magic Circle against Evil, works for anyone.
Other devine spells like Blasphemy, Unholy Blight, Holy Word, Sheild of Lathander work as per the evil/non-evil tags.

3] Druids are devine spell casters and armour does not affect their casting ability. Bards are arcane casters, even if they do get clerical healing spells. As such Bards are subject to spell failure if wearing armour or sheild.

4]Since these (L1) The Ballad of Three Heroes (L7) The Song of Kaudies (L9) The Siren’s Yearningare anyway useful only in combat, they aren't otherwise restricted.

The other three (L3) The Tale of Curran Strongheart (L5) Tymora’s Melody (L11) War Chant of Sithare useful in and out of battle. Of these only the last is restricted to exclusive use in battle.

In my game however it is bugged. Instead of affecting only my party, it effects everyone in the map. So everyone has +2AC, DR +2/-. Check to see if you have that bug. Because of this I have the mod which allows the song to be played outside of combat.

Thanks.

In my game however it is bugged. Instead of affecting only my party, it effects everyone in the map. So everyone has +2AC, DR +2/-. Check to see if you have that bug. Because of this I have the mod which allows the song to be played outside of combat.How do you know if it's affecting your enemies?

When I injure them, I get the message "x damage done to Troll (2 damage resisted)"
You will need to turn one the appropriate feedback options. While in-game press 'O' then the feedback button and choose all the options you want. I generally have all switched on.