Looked in the help already I assume, and found it not very?
Your #1 is a built-in function. I wrote a routine called 3D-Projector.lsp a
while back (with help from TomD) to project a point onto a given plane that
you might look at. It might give you an idea on how to try your #2. It's on
my site. It basically does some coordinate translation.
Click to see the full signature.