Why is there both 另外 and 别 in the sentence 你还是另外找别人吧, and why does 另外 come before 找?
It's a very hamfisted example of what the book is trying to illustrate -- that 另外 and 另 can both be used to modify a verb, but that 另 is restricted to modify only monosyllabic verbs. 

To answer the first part of your question: 另外/另 before 找 because it is specifically modifying the verb 找 using 另外/另 (note that you cannot use 另 for non-monosyllabic words according to the book).

The second part of your question involves the meaning of 别, which here, I believe you should take into account that 别 attaches to 人， so that you have 别人 as one word meaning "another person" (or more specifically "a person that is not already been mentioned")

In that case, 另外 is modifying 找, emphasizing that it is an additional and different task to be done, whereas 别人 mentions another person. I feel like it means something along the lines of "you should rather (do the additional task of) finding another person"