Isn't the fire damage a bit too overpowered? Especially considering the +50% elemental bonus from the Celestial realm – that's a total of +37% to overall effectiveness, no wonder everyone chooses it.

You can deal a lot of damage at once, or deal less damage more frequently, or live long and safely – it's all about balance.

What do action points affect? Is it only during peacetime?

Action points, surprisingly, affect the number of actions you can perform during a turn in combat.

And what type of damage does evasion apply to – arrows/physical/magical?

All types.

Why do different races have different total skill point values?

Because different people are naturally built differently, have different mentalities, and so on, and these parameters are completely disproportionate among them.

But the Elven race already has 67 points in total.

That's just how that race is; the description explains everything. In any case, it can only exist as a single unit in a group.

Can I take a half-breed Aristocrat-Elf?

Both options are already taken, but even if they weren't, it wouldn't be possible.

Can I create a Homunculus, add 10 points to Constitution, and the remaining 45 to Strength?

You can, but your character won't be able to talk or move.