So question, more about Elemental Spell in general that this particular combo, but Chill Touch was the best idea since it has multiple attacks as part of the cast.
For a prepared caster (Wizard), when do you decide that the spell is Elemental?
Obviously you need to prep it in a slot higher to apply the metamagic, but do you need to decide:
A: When the spell is prepared how much of it damage (half or full) will be the elemental type.
B: When you cast the spell, meaning you could make none of the spell's damage the elemental type if you needed the original type (though still using the higher level slot of course).
C: Before each attack with the spell. So a spell like Chill Touch, each attack could be none, half, or full the other Elemental type.
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Personally, I would rule B. You decide whether to apply the benefit of Elemental Spell when casting, and the higher level spell slot is giving you the option to split the elemental damage if advantageous to you rather than committing you to splitting the damage. Otherwise there's too much of a chance that you're investing a spell and a higher level spell slot and ending up with not just no benefit, but a worse spell.
Best rule support I have for that is the line "You make all pertinent decisions about a spell (range, target, area, effect, version, and so forth) when the spell comes into effect" and the fact that the feat says "You may replace a spell’s normal damage with that energy type or split the spell’s damage" rather than simply "You replace a spell's normal damage."
Wouldn't be surprised if I'm missing something and the answer is A, but I think the "make decisions about the spell when the spell comes into effect" would prevent you from changing the damage types round to round as per C.