And for number #5 you don't even need to write a function. Why not just use reverse? And notice his indentation and closing of parens is all wrong (i.e. not Lisp style). He's writing Lisp code thinking in a another language[1].
It's still a good exercise to go through them all as a Lisp newb like myself when I'm bored or need a break.
I just find it a bit interesting that you are supposed to implement things that are given. That sort of defeats the purpose of doing one for a specific language.
I think you don't really want to use length. Since it is a program evaluating last it might not be that bad as you have to goto the last element anyway.
Why not just something simple like.. (defun foo(thing) (list (nth (1- (length thing)) thing))
Same applies for problem 2.