[darkbreath]

Ok, I have some updates. First of all, as far as I can tell, the Reload button in Synaptic just does the equivalent of sudo apt-get update. The only way I can manually update the Xapian index is with sudo update-apt-xapian-index -vf. I guess one more command isn't too bad, and I only have to do it after adding a PPA, because I think the Xapian index automatically updates itself eventually. I don't know the interval, but I noticed that a newly added PPA had its packages listed eventually. Besides, I don't HAVE to use Synaptic; terminal works fine most of the time. I mainly use Synaptic to find package names when I don't know the exact spelling anyways.

I can also unfortunately confirm that sigil is not offered on Xenial. So basically on the latest LTS version of Ubuntu, the only way to install Sigil is by compiling from source at the moment. I managed to do it, but it certainly wasn't a walk in the park.