Optics question

I'm reading "Telescope Optics" by Rutten & van Venrooij and have
some questions.

First the observation that the authors appear wildly inconsistent
about what they think their target audience knows. I get the impression
the book most often assumes a knowledge of optics in general and
its goal is to flesh out telescope optics in particular with perhaps a brief
review.

In 4.2.2 they discuss coma and attribute it to "the intersection of
rays not being symmetrical." Shouldn't "off axis light" come into the
telescope in a symmetrical fashion when confronting an evenly distributed
light source? If one were to rotate the lens or mirror, would the coma
rotate as well?

WRT diffraction spikes, if the secondary mirror and struts are not in the
plane of focus, why would the diffraction effect occur where the mirror
itself is not visible? And if it's an "edge effect," way doesn't the edge of
the telescope tube diffract the light as well in a way that is visible?

Also, (and this may sound silly to those more knowledgeable than I)
isn't there a compound glass that has the property of being neutral to light
in one direction and reflect it 90 degrees from the other? It would be like
a two-way mirror with the mirror embedded into the prism at an angle.
It's such a simple solution to having an obstruction that this type of
prism must present insurmountable problems.

--
Craig Franck

Cortland, NY