This is all nice and good, but it does not override CRB p. 294.
You can resist spells even when there is no mage around. Spell resistance only includes counterspelling dice, if someone used spell defense. So since the title is "QUICK ANSWERS TO QUESTIONS ABOUT PREPARATIONS (PP. 304–306, SR5)" with Counterspelling not being applicable against preparations on p. 294, I'll reiterate one more time:
You are wrong.
You're right, absolutely right. The Counter Spell skill does nothing to the enchanted arrow.
Because it is just a an arrow with some magic. I need to the Disenchant skill for that.
But guess what happens when the arrow is triggered?
Here's a hint!
Once triggered, the spell cannot be stopped unless it is dispelled or its duration ends
Didn't catch that?
Once triggered, the spell cannot be stopped unless it is dispelled or its duration ends
Curious how it says... The spell isn't it? Not The preparation. Well, that's because its not a preparation anymore, it is now a spell. And..
Once the prepared spell is triggered, it can be dispelled or
resisted like any other spell
Ah there is "Spell" again! Lordy me, what a coincidence! Its almost as if..
ALCHEMY
From mystic flamethrowers to preparing a six-demon bag, alchemy allows the magician to prepare objects that hold a spell to be released later.
It is! It is! You're putting a spell in a object that gets released later. Kinda like well.. forgive my dnd terms here.. but perhaps a scroll? Maybe a pokeball if you're into that. Or perhaps a battery.
In fact
A preparation sustains the magic for the spell through the lynchpin (p. 304, SR5).
Geewillikers. Seems the effect that comes out when the preparation is triggered sure ain't no preparation still. In fact, everything seems to point to it being a spell!
And guess what!
Just like any other spell, it can be resisted or dispelled, because its a spell now. Not a preparation.
I'll reiterate once again, Counterspell and Dispel are not two different things. Rather Dispel is half of the use of Counterspell. The other half is Spell Defense, which applies to resisting spells.
And while you cannot dispel the object itself, you can dispel or resist the spell it creates. Just like how you can't dispel another mage. But you can dispel the spells he created. Or use spell defense against them.