Can a spellcaster let a target automatically succeed at its save to a harmful spell?

[Plausible Pseudonym]

Two items/spells in newly published player companions give partial effects on a successful save that can in narrow situations be better from the attacker's perspective than the effect on a failed save. Can I choose to let the target autosucceed at its save or somehow drastically lower the DC?

1. First consider this spell from Psychic Anthology.

Spoiler:

DEBILITATING PAIN
School enchantment (compulsion) [mind-affecting, painUM]; Level psychic 3, witch 3
Casting Time 1 standard action Components V, S, M
Range close (25 ft. + 5 ft./2 levels) Target one creature
Duration 1 round/3 levels
Saving Throw Will partial (see text); Spell Resistance yes
The target is overcome with intense pain, causing it to be stunned on a failed save or dazed for 1 round on a successful save.

Consider using this on something like an Elemental that is immune to stun, but not to compulsion, daze, or pain effects. They'd suffer no effect if they failed their save, but be dazed for 1 round if they succeeded. Can I ensure they succeed and counterintuitively take the worse result?

(And yes, this spell is an abomination that shouldn't have been published in this way, now anyone with a good spell resistance penetration can auto daze lockdown every powerful outsider that doesn't have a demigod's compulsion immunity.)

2. Next up is this item from Heroes of the High Court.

Spoiler:

CHASTISING BATON
SLOT none
AURA moderate necromancy
CL 7th
PRICE
5,000 GP WEIGHT 3 lbs.
This short metal rod is etched with the outlines of various weapons and is usually owned by strong-armed rulers who keep a tight rein on their nation’s military. A chastising baton adds 1 to the saving throw DC of any compulsion spell cast by the wielder and adds the pain descriptor to the spell. Creatures that succeed at a Will save against a compulsion spell cast by the wielder are racked with pain, taking 1d6 points of nonlethal damage and becoming sickened for 1 round.
CONSTRUCTION REQUIREMENTS
COST 2,500 gp
Craft Rod, pain strike

This combos very well with Ego Whip targeting Wisdom to apply a Will save penalty. If you fail against Ego
Whip you are staggered for a random number of rounds and suffer the Will save penalty for rounds/CL. But if you succeed at the save you still take the Will save penalty for 1 round. Add in this rod and you tack on an extra -2 sickened penalty for one round.

Compare this rod and Ego Whip V (7th level spell) to Limited Wish (7th level spell) used to inflict a -7 penalty to the targets next save. Ego Whip would inflict a -7 Will save only penalty vs. the -7 to any save that LW provides, but it lasts a full round, not just the next save, so you can better coordinate/pack attack with your party. Plus Ego Whip is free and LW costs 1,500. And as a minor bonus for Ego Whip and the rod, a successful save would further debuff the target outside its saves via the sickened penalty to attacks, damage, skill checks, etc.

So are there rules for casting a harmful spell that you want the target to mostly resist and only take the partial result?

[Ring_of_Gyges]

RAW? Nope, your DCs are what they are and people make the saves or not based on their rolls.

Personally? I would rule the same way I do when fighters want to do less damage than they're entitled to.

I would say that the base DC of 10 + spell level is non-optional, but that you can apply as much or as little of your stat bonus, bonus from feats or class abilities, and so on as you like.

So, if you're normally DC 22 for 10 base + level 5 + 5 int + greater spell focus, you could cast the spell at anything from a DC 15 to 22 to resist.

That said a feat that lets you voluntarily lower the DCs of your spells seems fine.

[Darrell Impey UK]

Should you choose to allow it it would have to be an everyone or no one thing; dropping the DC for all your allies within an area effect to 2 whilst keeping all your enemies at high numbers is just too powerful.

[Qaianna]

I'm surprised that immune to stun doesn't include immune to daze, personally.

[Plausible Pseudonym]

I feel certain that would be a Pathfinder 2 rule.