[Kusaja]

It's hard for me to honestly rate the episode without understanding any significant amount of Japanese at all, just reading through online summaries, but for now I'd have to say that it seems to be quite good.

It doesn't advance the main Britannia plot, no, but it does have some apparently interesting Lelouch, / C.C. character moments (and Viletta fanservice, of course).

As for Mao's situation...

Spoiler:

There's also a chance that he might actually have died in this episode, but not without making "prior arrangements" against Lelouch and Nunally. That would mean that he took action before his death, setting up whatever specific incident happens in episode 16. In other words, he planned ahead of this apparent death.

Of course, Sunrise might not be smart enough to pull something like that off, but I'd prefer that to the implied "resurrection" that everyone's assuming to be the case (I've read the spoilers too, but even those don't go into too much detail about anything).