trust me, it's come up a number of times before.
The major issue has always been that the spell must be cast before the attack, which seems to imply a need to either cast it in an earlier phase, or to have initiative and cast it before the attack in the same phase.
The errata states the spell must resolve within the round.
IMO the confusion is due to these spells text not being updated since the RM2 versions. . .in which spell phase always came before missiles or melee. . .so the spell resolved within the round, and was always declared and cast before the attacks it affected. . .since the order of actions was fixed in RM2, there was no need to address it as an interrupt.
Also quite obviously the spells were cast as a "I will choose to deflect/turn/untrue one attack this round", since no attack could even be declared until after spell results phase, so there was no way of knowing on spell phase who would be attacking where.
As to the anti spell spells. . .there's no initiative in RM2 spell phase if I recall correctly, so all spells within a round resolved simultaneously. . .also making it easy and clear if you cast on me, and I cast a spell against your spell, your spell has to deal with my counter.
Ideally, the more open round of RMSS would have provoked an updating of those spells to clarify all the potential order of action issues due to it now mattering who went first. . .but insofar as I'm aware, that never happened.