Thanks for the reply.
So it's an imperative here? I can see it work in the first 2 sentences, but not so much in the 3rd one, though. Wouldn't it be more like "Wizards always (try to) keep multiple options (for doing something) in mind."? Just because there's a subject (魔法使いたる者) and lack thereof (i.e., only implicit -you-) in the previous 2 sentences. I could be wrong, but In the first 2, the manual is "telling you" with a light imperative, as you said, but the 3rd seems to be describing what wizards tend to do. Any thoughts?
Also, just to confirm, is the stuff I said about ~ように。 correct?