Hello Jay, welcome to the forum!
I am not sure how much Morrus has worked on the concept since the post you replied to was dated back in 2014. The WOIN website portion on magic doesn't reference the additional magic skills of scry or counterspelling.
But to pick up the conversation from where it left off.. I think counterspelling is a cool option that can make games more entertaining... but getting the balance between frustrating players and not becoming overpowered.
I have yet to see an RPG system do counterspelling well. The closest I have seen is basically a mana-point bidding war between the original caster and the attempted counterspell.
In WOIN framework, perhaps a roll-off in which each mage gets 1 die per mana-point. the casting mage starts with the pool equal to the mana spent to cast the spell. The opposing mage builds a dice pool by burning mana.
Both sides add dice based on their counter-spelling skill and any implements that specifically assist that skill.
Each side rolls the dice-pool in secret and can burn mana to add more dice.
Once both sides stop adding dice, the mage with the highest total has control over the spells target.. or can fizzle it.
You have to know one of the secrets of the spell to be able to re-target the spell. If you don't know any of the secrets you can only fizzle the spell.
Countering a spell could be either a readied action {makes it less likely to be used but more of a tactical choice} or a reaction that burns your next normal action.
An alternate method within the EOM framework would be a reaction casting of an hex that increases the difficulty of casting the spell {malus to targeting roll or burn additional mana to cast normally?}
Anyway, enough of my ramblings. Morrus should be along soon to say whats what