I'm guessing you didn't want to wait for a reply here and posted over there too.
But yes, it is all magery.
If you have enough magery to get all the properties, you have the same chance of a spellbook you craft having them with 50 inscription as with 100.
(you just waste a lot of scrolls getting successes making the books at 50 skill)
Scrappers are the opposite. Inscription decides the exceptional chance, and only exceptional scrappers get bonus props.
Just a little extra info..
Slayer property is actually the First one checked in every case. If the check results in a book with a slayer property, the other 2 chances are noticably reduced.
So the odds of getting a Slayer + 2 property book are pretty small.
I've burned about 250k scrolls here and there and still shooting for that SDI/MR/Dragon Slayer.
On the bright side, you can often find scrolls in Felucca for 3g per, and wizard crystal reading glasses sell for enough to recoup most of your costs.