Answer to the first question: yes, reflexive pronouns can follow prepositions; just as in English by the way
He is angry with himself.
As for the second part:
First of, we have to make a distinction between pronouns that simply refer back to the subject and such that relate individuals of a group subject to each other. The latter are called "reciprocal" and "einander" is an example for those.
reflexive pronouns (selection): myself, yourself, ourselves...
reciprocal pronouns: each other, one another
There is a difference in meaning and so you cannot always replace "ourselves" with "einander".
The second "rule" in your book is wrong.
Die Spieler staunen über sich.
Die Spieler staunen über einander.
The first sentence means that the players as a whole are amazed by themselves as a whole.... like... "Wow, we're a great team." The second means that the players are amazed by each other... like ... "Wow, Thomas you were amazing." "And you, Steve, and you."
If what you want to express is the reciprocal meaning then it is considered good style to use "einander" because "sich" is ambiguous.