Can I stop a Gentoo "emerge world" to add a feature or use flag I forgot? If so, how much pain am I in for, and what steps would you recommend for safely getting the system back on its feet?
It is perfectly safe...
but, when you change global USE flags. You may as well run a
emerge -auDN world. That'll re-emerge anything that was built with different flags than what you have now.
When that is finished, it is also often a good idea to do a
gentoolkit to ensure that libraries are in good shape.
Finally, when all this is done, you can optionally do a
emerge -pv --depclean to remove any packages which are no longer required.
yes, you can stop an emerge with Ctrl+C at anytime. if emerge is on the compile phase, there's no problem, "make" and "make install" run in a separate directory. but if emerge is on the final "installing..." phase, when it copies the installed files to the system, some of the files may have been already merged, and that may cause an inconsistency; but if you're going to change the USE flags and reemerge the package, that's fine too.
so, in summary, you can stop an "emerge world" anytime, it won't mess up with your system.