1. See this thread for the info on the DYLD_FALLBACK [afni.nimh.nih.gov]
2. I haven't used tcsh in about 10 years, I'll defer to someone else on those specific questions.
3. You do not need to install Xcode to run AFNI... BUT I would really suggest you have it handy for users who want to do more than scripting commands together. If you're installing 10.7.x, I'd say Xcode 4.5. Don't forget to install the command line tools.

One thing to consider in using the user .profile vs. the global profile is that if you are deploying with Mac OS X Server, you can automatically set users to include a default .login or .profile (or .bashrc, etc) when they login for the first time. In general I would hesitate to add it to a global path as users may want to use a different version of AFNI from what is installed. You might also consider installing AFNI in /usr/local/abin or /Applications instead of at the root directory (/), just to keep with some conventions.