Conclusion
There you go, a somewhat geeky and definitely not trivial recipe to happy Linuxing with Scientific. It pains me that repository conflicts can exist and come to bear in such a trivial manner, but this is something that distro developers need to sort. The best solution would be to make the official repositories as rich as possible, so they include everything. Then, there will be no need for dangerous tweaking.
On the bright side, you learned about smart software management, how to digest problems one bite at a time, enabling repositories in an automated manner and manually, importing GPG keys, protecting your repositories, prioritizing updates, and more. Hopefully, this was a valuable lesson. Have fun, children of the Internets!
P.S. This article is also applicable for RHEL, CentOS and Fedora users!
Cheers.