By the same token, the bonus from spells like Bladeturn and Deflections was set so high (-100 to the attack) that it seems clear they were in fact intended to virtually completely negate an attack, and were thus intended to be almost your whole activity for the turn; they weren't intended to be taken with a full OB attack on top. They saved your life-- they didn't allow you to do two full actions (entirely negating an opponent's attack and getting a full attack of your own) at once.
I dont really see a problem here. Pure Arms user can use Adrenal Move Speed to get 2 attacks out (and those can be used to eat up an opponents parry and then get a "free" swing, or parry and attack) or just come with multiple attacks right away. Even the combination is possible. Usual practice for Martial Arts users (4 attacks wohoah).
So why not give a semi spell user or any character with the support of a magic user the chance to magically turn one of those attacks and maybe parry the other and see another day. It again burns magic points fast (Bladeturn/Deflection are considerably higher level spells) and if not cast on self but others basically takes the magic user out of the offensive.
We never really encounterd an instant spell, which should be slower. They are usually utility/defensive. If not, we figured they should not normal spells (Adrenal Focus of the NIghtblade, would not really work as a regular spell, ass the +attack Spells).
Sometimes having the Instant* Spell on a Base List is from my point of view the gadget of that given class, compared to non instant,in effect comparable, spells on open/closed lists. I have no example right now by heart, but I remember that we looked into comparisons like that several times in the last 35 years.
I personally think its kind of fun, if the usual semi spell user, who finds himself in as surprising combat situation, needs some rounds to get the defense and support for buddies up (Shield, Blur, Aura.... ). Makes a big difference to the prepared situation where the situations turns around and a pure arms opponent better parries till some gadget spells wear off the first time.