How can I identify which spells are incompatible with Merciful Spell?

Question

The answers to this question indicate that the Merciful Spell feat can't be used on Summon Monster spells, since they don't deal damage directly. This raises the question of which other spells are also incompatible with Merciful Spell. For example:

• Can you create a Merciful Spike Growth? Unlike Summon Monster, it mentions the damage that it inflicts in the spell description. However, the damage is still indirect, and I'm pretty confident that Merciful Create Pit doesn't work.
• Can you summon a Merciful Spiritual Weapon? Like Summon Monster, it isn't doing direct damage. On the other hand, there is the Merciful weapon enchantment, and you can strike non-lethally even with a normal weapon.
• Can you cast a Merciful Acid Arrow? It's similar to a direct damage spell, but it's still a conjuration (creation) effect. Also, since the acid lasts several rounds, in a sense it's like trying to inflict nonlethal damage with an Alchemist's Fire.
• How about Merciful Sirocco? The damage is ongoing and arguably an environmental effect, but as an evocation spell it's arguably more like Fireball than Create Pit.

An ideal answer would cite or suggest some kind of general rule for identifying incompatible spells, such as "Merciful Spell only works on instantaneous spells" or "Merciful Spell doesn't work on spells that bypass spell resistance". However, answers for these specific spells would still be helpful, since they would provide at least some guidance.

Answer 1

Spells that directly deal damage can be made merciful.

Merciful Spell says (emphasis mine):

You can alter spells that inflict damage to inflict nonlethal damage instead. Spells that inflict damage of a particular type (such as fire) inflict nonlethal damage of that same type. A merciful spell does not use up a higher-level spell slot than the spell’s actual level.

If the spell directly deals damage, as is the case of each of those 4 spells, merciful works on that damage. In the case of Create Pit and Summon Monster, the spells themselves don't deal damage. In create pit's case, it's the falling rules that do, and in Summon Monster's case it's the summons that deal damage, not the spell itself.

More examples:

• Acid, Hungry, and Spiked Pit can be made merciful, but the nonlethal damage would only apply to the damage the spell deals, not the damage from falling.
• Dispel Magic on a creature that's flying through the use of a Fly spell cannot be made merciful, because dispel magic doesn't deal damage.
• Similarly, you can't make Reverse Gravity into a merciful spell, the spell itself does not deal damage, only the falling rules for when a creature slams into the ceiling does.
• Instant Weapon cannot be made into a merciful spell, as it's only effect is the weapon it makes. Spiritual Weapon can, because the spell itself is dealing damage, despite the requirement of an attack roll.
• Flame Blade can also be made a merciful spell, despite the similarity it shares with instant weapon. Although it makes a weapon for you to wield, unlike instant weapon, the damage is a result of the spell, not the weapon itself.

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Side note: Technically any spell can be made into merciful spell (it actually doesn't have any requirements for what spells it can be put on), it's just that it has no effect on certain spells. So when I say that a spell can't be made into a merciful spell, I mean that there is no difference between the regular version of that spell, and a merciful version of that spell.