People are probably getting sick of my bringing this up over and over, but I have a somewhat paradigm solution for a part of that question: The Law of Spontaneous Generation.
It is an argument against requisites, so beware. Basically, the law is this: A magical effect can spawn natural effects without the necessity of pre-desposing that magical effect towards that end. To give an example-- If you create a log, CrHe10, and throw it in fire, it will burn, i.e. produce flame, even though you didn't use an Ignem requisite. Another, more controversal example: If you create a cloud, CrAu, of sufficient size, it will spontaneously generate rain via nature's rules, that clouds of huge size make rain, without an Aquam requisite. After all, you don't have to put an Ignem requisite in Incantation of Lightning, despite the fact that it causes small fires. You don't have to predespose the cloud to be a 'rain'cloud by putting an Aquam requisite in your spell. You certainly can, if you want, but it isn't necessary. Air can create water naturally, via spontaneous generation, just as Herbam can create Animal naturally, via spontaneous generation (rotting logs producing termites, for instance).
Another point to the Law of Spontaneous Generation: Causality should not be undone by the non-permanence of magical effects. If you create a horse, and it disappears at sunset, all the tracks it made don't disappear. The fact that you were able to ride it doesn't disappear. So the fact that it impregnated another horse shouldn't disappear. Objects in nature have the ability to spawn effects, and the fact that they do has nothing to do with their permanence on this earth.
So, a temporary stallion can impregnate a mare, disappear at sunset, and the mare could still give birth. What the essential nature of the stallion was that is passed onto the mare is up to you people-- a mouse/horse crossbreed is gonna be weird. But the fact that the mare was impregnated should not be undone, because causality should be preserved. No matter what, the mare has to continue like she was impregnated. Now, if you want to rule that the disappearance of the stallion weakens the sperm he generated, I would say that the sperm was naturally generated by the horse, like its hoofprints, and so exists despite the non-existance of that which gave it life. I don't believe that the mare would necessarily have a stillborn/miscarriaged foal because the father was a magical effect.
Look at it this way. If you Creo a horse, right, and it eats grass, that grass will still be eaten at sunset, right? It doesn't reappear. So by further extension, if the horse digested that grass fully, and excreted it back out, as either urine or manure, that urine and manure stick around after sunset, right? So the horse, a non-permanent CrAn effect, generated Aquam and Herbam permanently-- via nature and spontaneous generation. So too should that horse be able to generate semen and sperm, and thus another horse entire, if a permanent mare was about.
Don't let the 'No Creo effect may be permanent without vis' rule get to you. It has boundaries, like any rule. You can create things permanently without the use of vis, if you do so indirectly and with the assistance of nature. If I have a mare, a lot of time and horsefeed, and a CrAn skill that's high enough, I can create another horse permanently without vis. What the foal will be like, who can say, but you can do it, just as you can get around the Parma Magica indirectly with natural things. If you take the rule too far, you can destroy causality and sanity by retroactively undoing history. Spontaneous generation fixes this.
This was all IMHO, of course, and subject to Robbie's Mantra.
Sgt. Brickman, Paradigm Police Badge #21