Honestly, the Shadow _____ series of spells are some of the most poorly written spells in the PHB. You are correct in that it works ok with regard to shadow evocations that do directed damage like Shadow Evocation fireballs and lightning bolts. Problems arise with emulating other evocation spells or when trying to emulate some other property besides dice of damage that the spell inflicts. Shadow conjuration spells are even worse.
This doesn't ansewer your question but I understand where your questions come from. Personally I banned the shadow_______ spells in my games until I get an official ruling or an errata on how they operate. I had a player who used them regularly and they caused so many headaches that I got sick of the spells. The authors simply imported these spells from earlier editions without ever bothering to clarify exactly how they operate.
I'll give you example:
Postulate for a moment that lamp oil always ignites when exposed to fire. Does lamp oil ignite when exposed to a shadow magic fire? The oil has no ability to believe in the shadow illusion so is it fire with regard to the oil? If you decide that the oil does ignite, then why and what is the oil burning? Is is fire or is it shadow-fire?
If you have a protection from fire spell active, does that absorb a shadow-magic fireball if you believe that it is fire? What if you disbelieve the illusion? Does a person's perception of whether the illusion is real or not affect the protection spell, after all, once the spell is cast he has not power to change it so why should his perception of whether or not the fire is real affect the spell?
If you cast a shadow-magic flamestrike, is 1/2 of the damage holy? What if you disbelieve the illusion, is 1/2 of the remaining shadowstuff damage (which you now know is not fire) holy since it isn't strictly speaking an energy type?
The same problem comes up with shadow conjured matter. If you make a shadow wall of iron, is it iron to something that can't believe in it such as a magnet? If it is iron, it is magetic, if not then it isn't? What if you cast the Magnetism spell from OA on a wall you believe to be iron? Again, does your belief that something is real translate to your spell which has no independent capacity to believe and is no longer subject to your control or belief once it leaves your hand. How about an arrow that your fire from a bow? Does it clank against iron or sink into shadowstuff?
Also, what does __% real mean exactly. Does that wall of iron only have __% of the hp, is it __% thick, is it __% magnetic, does it only weigh __% as much? Which of the above?
These spells are so poorly thought out that is is maddening.
My interpretation is that a Shadow _____ spell is not the same as a the standard spell. Daylight cancels Darkness, shadow daylight does not cancel darkness. Similarly an Iron Golem that is slowed by electricity is not slowed by shadow magic imitating electricity because it is not electricity it is shadowstuff that is trying to imitate electricity.
Has The Sage or any official errata ever made a clarificaiton on these spells? (God I hope these spells get either fixed or nerfed completely in 3.5e.)
Tzarevitch