Yikes, that sure is a long and aggressive post.
The idea is that our wizards would use the spell from a distance, that shadows are as strong as a person (or strong enough to hold a dagger, which is all is really needed to assasinate enemies) and that shadows look like like actual shadows when intangible. That hardly seems like overreach.
So yeah, if the shadow can only go ten feet, then it's useless, but so is any spell that can only go a short range. If that was an issue then we couldn't research things like magic missile. A ten foot magic missile range is not as strong as a 100 foot magic missile range.
And if shadows aren't stealthy or able to be used for combat, then it's usefulness is also decreased, but again, we take similar risks with any spell. To use the magic missile as an example again: its components are range and damage. If it lost one of those, it would be a bad spell. Shadows components are range, damage, and stealth. So sure, it's more complex than a basic spell, but you're blowing it way way way out of proportion here. It doesn't even need to be a low level spell, it could be a mid level spell and still be useful as it would only take one shadow to sneak into an enemy camp with a dagger, kill their leaders and wizards in their sleep, grab the enemy spellbook, and return home.
The same effect could be achieved with a spell of stealth or invisibility.
Our issue on the plains seems to be manuverability. According to the OP, our army is a loose collection of bands of fighters with low mobility. The enemy has highly mobile troops and Calvary, as well as better formations. So we're at a severe disadvantage there.
But I'm not sure we don't already have stirrups. We couldn't really have Calvary without them as they're an integral part of horse riding. And if they haven't been invented yet, we should be knocking enemies off their horses left and right (which isn't the case). But if the case is that stirrups haven't been invented in this universe, then sure, that's a pretty useful invention.
Of all the ideas discussed here, perhaps RAMs summon spear wall would be the most useful. Or even a variation on that that's just a straightforward summon magic lance which is basically the same thing, but probably easier to use and more flexible (a good wizard can summon a spear wall to stop a charge, verses several weak wizards conjuring lances to stop charges and skewer enemies in open combat too). Having a straightforward "Kill the Bad Guys" kind of spell like that wouldn't hurt. Even if it's close range, it should be fairly devastating against their lightly armored men and horses.
So I guess I'm changing my vote again: Magic lance spell, final answer.