[RPG] Helpless opponent & Coup de Grace

This is not strictly RAW, but it is from an article called Rules of the Game: Actions (Part Two) by Skip Williams on the WotC website.

It's usually best to allow... opposed checks to resist being tripped or grappled, unless they're helpless (any ability score reduced to 0, paralyzed, petrified, or unconscious). It's worth noting, however, that even a helpless character can make a Reflex save (albeit with a -5 penalty for an effective Dexterity score of 0). That's because the game generally favors at least some involvement from the player -- and a chance for a miraculous escape -- whenever a character is in danger.

So Jonathan Drain's comment is dead on. The saving throw doesn't map directly to dodging out of the way, it's a metagame mechanic. A good way to think of it is that the "reflex" part of the save is best used after the roll to narrate how the character avoided death.

Let's check the definitions.

Death attack (emphasis mine):

If an assassin studies his victim for 3 rounds and then makes a sneak attack with a melee weapon that successfully deals damage.

Coup de grace:

As a full-round action, you can use a melee weapon to deliver a coup de grace to a helpless opponent. You can also use a bow or crossbow, provided you are adjacent to the target.

Therefore

• Yes, assassin does trigger Death Attack when makes coup de grace, provided he makes it with a melee weapon.
• RAW, no. Complete Arcane (pp 85-86) tells us nothing about weaponlike spells and death attacks/coup de graces, and it features a list of things you can do with weaponlike spell. Wouldn't hurt to houserule otherwise, since you can sneak attack or critical hit with a weaponlike spell. FAQ agrees on this matter, albeit it mentions rays only:

Can you perform a coup de grace with a ray spell?

No. The coup de grace action specifically allows you to use a bow or crossbow, provided you are adjacent to the target (PH 154).

That said, allowing spells with a ranged touch attack to deliver a coup de grace against an adjacent target wouldn’t be an unreasonable house rule.

Complete Mage, Spell Compendium and Rules Compendium have no additional information on the subject.

Exception:

Spellwarp Sniper 5 gains the ability to coup the grace with a ray spell that deals hit point damage.

Ray Mastery You can deliver a coup de grace with a ray spell that deals hit point damage .You must be adjacent to your target to deliver the coup de grace .

This ability still doesn't allow to trigger Death Attack.